Water-soluble propylene-maleic interpolymers containing recurring units having the formula: <FORM:0871589/IV (a)/1> in which one Y is a hydrogen atom and the other Y is either a hydrogen atom or is a group derived from an alkanol Y,OH, either with or without recurring units having the formula: <FORM:0871589/IV (a)/2> in which Y is as defined above, and having a specific viscosity of at least 0,1 when determined in a 1% by weight solution in dimethyl formamide at 25 DEG C. when both Y's are hydrogen atoms, or being obtainable by partial esterification of such an interpolymer when one Y is derived from an alkanol, are made (1) by polymerizing propylene, or mixtures of propylene and ethylene, with maleic anhydride, the product being hydrolysed to the free acid form or the acid esters then being obtained by partially esterifying with an alkanol, preferably a low molecular weight alkanol, e.g. methanol, ethanol, isopropanol, n-butanol, secondary-butanol, tertiary butanol, isobutanol or one of the amyl alcohols, or a higher primary, secondary or tertiary alkanol containing 8-20 carbon atoms, or (2) by polymerizing propylene, or mixtures of propylene and ethylene, with maleic anhydride plus one or more monoalkyl maleates, or with one or more monoalkyl maleates in the absence of maleic anhydride, the reaction being preferably effected in either case in the presence of a liquid organic diluent and a free radical-liberating catalyst, e.g. benzoyl peroxide, 2, 4-dichlorobenzoyl peroxide, tertiary butyl peroxide, tertiary butyl hydroperoxide, diacetyl peroxide, diethyl peroxy carbonate, dimethylphenylhydroperoxymethane (cumene hydroperoxide), hydrogen peroxide, potassium persulphate, a .a 1-azodi-isobutyronitrile, azomethane or diazoamino benzene. The hydrolysis of the interpolymer may be effected by contacting a slurry thereof in an organic solvent with water by mixing the interpolymer with water or by subjecting the interpolymer to the action of water vapour. Interpolymerizations with maleic anhydride are preferably carried out at 40 DEG -90 DEG C. at super atmospheric pressures in the presence of a solvent for the maleic anhydride which is preferably a non-solvent for the interpolymer, such as an aliphatic or aromatic hydrocarbon or a chlorinated hydrocarbon, e.g. benzene, toluene, xylene, n-hexane, mixed hexanes, octane or ethylene dichloride. Low molecular weight interpolymers may be obtained by carrying out the interpolymerization in the presence of (1) an aldehyde having the formula R,CHO where R is a hydrogen atom, a hydrocarbon group or an aldehyde-substituted hydrocarbon group, or (2) a dihydrogen or monohydrogen phosphite having one or two monovalent hydrocarbon groups, e.g. dibutyl hydrogen phosphite, or (3) a mixture of benzene, a halobenzene or haloparaffin with an alkylated aromatic hydrocarbon having at least one a -hydrogen atom, e.g. a mixture of benzene and ethyl benzene or paracymene or of ethylene dichloride and di-isopropyl benzene. The products are useful for sizing nylon yarns (see Group IV(c)). Specifications 461,236, 461,237, and U.S.A. Specifications 2,071,250, 2,130,532 and 2,130,948 also are referred to.ALSO:Nylon yarns are sized with an aqueous solution of a water-soluble propylene-maleic interpolymer containing recurring units having the formula:- <FORM:0871589/IV(c)/1> in which one Y is a hydrogen atom and the other Y is either a hydrogen atom or is a group derived from an alkanol Y,OH, either with or without recurring units having the formula:- <FORM:0871589/IV(c)/2> in which Y is as defined above, the interpolymer having a specific viscosity of at least 0,1 when determined in a 1% by weight solution in dimethyl formamide at 25 DEG C. when both Ys are hydrogen atoms, or being obtainable by partial esterification of such an interpolymer when one Y is derived from an alkanol; the yarns are then dried. The interpolymers may be prepared (1) by polymerizing propylene, or mixtures of propylene and ethylene, with maleic anhydride, the product being hydrolysed to the free acid form or the acid esters, if these are required, being obtained by esterifying with an alkanol, preferably a low molecular weight alkanol, e.g. methanol, ethanol, isopropanol, n-butanol, secondary-butanol, tertiary-butanol, isobutanol or one of the amyl alcohols, or a higher primary, secondary or tertiary alkanol containing 8-20 carbon atoms, or (2) by polymerizing propylene, or mixtures of propylene and ethylene, with maleic anhydride plus one or more monoalkyl maleates, or with one or more monoalkyl maleates in the absence of maleic anhydride, the reaction being preferably effected in either case in the presence of a liquid organic diluent and a free-radical-liberating catalyst, preferably a peroxide- or azo-type polymerization catalyst (see Group IV(a)). The size solution preferably contains 2-15% by weight of the inter polymer and is preferably applied to the yarn at a temperature of 100-200 DEG F. in an amount and in a concentration sufficient to deposit 0,5-10% by weight, based on the dry weight of the yarn, of the interpolymer on the yarn. The solution may also contain small amounts, e.g. 0,01-5,0% by weight, of sizing adjuncts, p e.g. urea, humectants, oils and wetting agents. The sized yarns may be air-dried at 65 DEG -85 DEG F. or oven-dried at 140 DEG -250 DEG F. Specifications 461,236, 461,237 and U.S.A. Specifications 2,071,250, 2,130,532 and 2,130,948 are referred to.