Fibrous substrates are treated with (a) at least one oligomer of branched or straight-chain C3-C10-alkene having an average molecular weight Mn of up to 1200 g/mol, (b) at least one emulsifier which is obtainable by copolymerization of (A) at least one ethylenically unsaturated dicarboxylic anhydride, derived from at least one dicarboxylic acid of 4 to 8 carbon atoms, (B) at least one oligomer of branched or straight-chain C3-C10-alkene, at least one oligomer having an average molecular weight Mn of up to 1200 g/mol, (C) at least one α-olefin of up to 16 carbon atoms and (D) optionally at least one ethylenically unsaturated comonomer, and optionally reaction with (E) at least one compound of the formula I a, I b, I c or I d and, if appropriate, subsequent contact with water, where, in formulae I a to I d, A1 are identical or different C2-C20-alkylene, R1 is linear or branched C1-C30-alkyl, phenyl or hydrogen, and n is an integer from 1 to 200.