Cosmetic process for treating keratin materials, comprising the topical application to the keratin materials of a composition comprising, in a physiologically acceptable medium, a phosphonic polymer derived from the polymerization of: (a) 45% to 95% by weight, relative to the total weight of monomers, of an ethylenic monomer bearing an at least C 8 linear or branched alkyl group; (b) 5% to 25% by weight of vinylphosphonic acid monomer of formula; (c) 0% to 50% by weight of additional monomer chosen from: (i) linear or branched C 1 -C 6 alkyl (meth)acrylate or C 6 -C 12 cycloalkyl (meth)acrylate non-silicone monomers; (ii) polydimethylsiloxane silicone monomers bearing a mono(meth)acryloyloxy end group of formula below: in which: - R 8 denotes a hydrogen atom or a methyl group; - Rg denotes a linear or branched divalent hydrocarbon-based group containing from 1 to 10 carbon atoms and optionally containing one or two ether bonds -O-; - R 10 denotes a linear or branched alkyl group containing from 1 to 10 carbon atoms and especially from 2 to 8 carbon atoms; - n denotes an integer ranging from 1 to 300; said vinylphosphonic acid monomer of formula being: in which: R1 denotes H or -CH 3; X denotes a covalent bond and n denotes an integer ranging from 0 to 14; or X denotes a -COO- group and n denotes an integer ranging from 2 to 6. (a) 45% to 95% by weight, relative to the total weight of monomers, of an ethylenic monomer bearing an at least C 8 linear or branched alkyl group; (b) 5% to 25% by weight of vinylphosphonic acid monomer of formula; (c) 0% to 50% by weight of additional monomer chosen from: (i) linear or branched C 1 -C 6 alkyl (meth)acrylate or C 6 -C 12 cycloalkyl (meth)acrylate non-silicone monomers; (ii) polydimethylsiloxane silicone monomers bearing a mono(meth)acryloyloxy end group of formula below: in which: - R 8 denotes a hydrogen atom or a methyl group; - Rg denotes a linear or branched divalent hydrocarbon-based group containing from 1 to 10 carbon atoms and optionally containing one or two ether bonds -O-; - R 10 denotes a linear or branched alkyl group containing from 1 to 10 carbon atoms and especially from 2 to 8 carbon atoms; - n denotes an integer ranging from 1 to 300; said vinylphosphonic acid monomer of formula being: in which: R1 denotes H or -CH 3; X denotes a covalent bond and n denotes an integer ranging from 0 to 14; or X denotes a -COO- group and n denotes an integer ranging from 2 to 6. (i) linear or branched C 1 -C 6 alkyl (meth)acrylate or C 6 -C 12 cycloalkyl (meth)acrylate non-silicone monomers; (ii) polydimethylsiloxane silicone monomers bearing a mono(meth)acryloyloxy end group of formula below: in which: - R 8 denotes a hydrogen atom or a methyl group; - Rg denotes a linear or branched divalent hydrocarbon-based group containing from 1 to 10 carbon atoms and optionally containing one or two ether bonds -O-; - R 10 denotes a linear or branched alkyl group containing from 1 to 10 carbon atoms and especially from 2 to 8 carbon atoms; - n denotes an integer ranging from 1 to 300; said vinylphosphonic acid monomer of formula being: in which: R1 denotes H or -CH 3; X denotes a covalent bond and n denotes an integer ranging from 0 to 14; or X denotes a -COO- group and n denotes an integer ranging from 2 to 6. - R 8 denotes a hydrogen atom or a methyl group; - Rg denotes a linear or branched divalent hydrocarbon-based group containing from 1 to 10 carbon atoms and optionally containing one or two ether bonds -O-; - R 10 denotes a linear or branched alkyl group containing from 1 to 10 carbon atoms and especially from 2 to 8 carbon atoms; - n denotes an integer ranging from 1 to 300; R1 denotes H or -CH 3; X denotes a covalent bond and n denotes an integer ranging from 0 to 14; or X denotes a -COO- group and n denotes an integer ranging from 2 to 6.