Abstract: The invention relates to new cross-linked copolymers containing carboxyl groups and especially useful as thickening agents for aqueous and organic solvents, which copolymers contain(A) 20 to 100% by weight of olefinically unsaturated carboxylic acids,(B) 0 to 80% by weight of at least one mono-olefinically unsaturated monomer copolymerizable with the components A and C, and(C) 0.1 to 4% by weight, calculated on the total quantity of monomers, of allyl monomer of the general formula ##STR1## in which n=0, 1 or 2,m=1, 2 or 3, wherein n+m=3,R.sub.1 =hydrogen, or an alkyl, alkoxyalkyl, haloalkyl, cycloalkyl, aryl, alkaryl, aralkyl or alkenyl radical, andR.sub.2 and R.sub.3, which may be the same or different, each represents hydrogen or an allyl, alkyl, cycloalkyl, aryl, aralkyl or alkaryl radical,wherein at least two of the radicals R.sub.1, R.sub.2 and R.sub.3 must be an allyl radical or a substituted allyl radical.