The invention relates to a cosmetic process for treating keratin fibres, comprising: (i) a step of applying to the keratin fibres a cosmetic composition containing a polyvinyl alcohol polymer comprising:—；an alcohol unit—；optionally an acetate unit—；a photocrosslinkable unit—；a hydrophobic unit； (ii) a step of irradiating the composition on the keratin fibres to crosslink said polymer. The invention also relates to the novel polyvinyl alcohol polymer used in said process. The treated keratin fibres have good cosmetic properties in terms of a soft feel and disentangling, which properties are persistent after one or more shampoo washes.
