Anionic sizing agents containing anhydride groups, which are novel compounds and which contain a single N,N-C.sub.6 -C.sub.22 di(alkyl- or alkenyl)amido group, a N-C.sub.6 -C.sub.22 (alkyl- or alkenyl)amido group or a C.sub.6 -C.sub.22 -(alkyl or alkenyl) ester group as hydrophobic substituent, and at least one anionic group in acid or salt form, in particular the anhydride of a N,N-C.sub.6 -C.sub.22 dialkylamido- or N,N-C.sub.6 -C.sub.22 dialkenylamidocarboxylic acid, the anhydride of a N-C.sub.6 -C.sub.22 alkylamido- or N,N-C.sub.6 -C.sub.22 alkenylamidocarboxylic acid, or the C.sub.6 -C.sub.22 alkyl ester or C.sub.6 -C.sub.22 alkenyl ester of an aromatic tetracarboxylic acid anhydride or a salt thereof, are particularly suitable, together with commerically available retention aids, for use in a process for pulp-sizing or surface-sizing paper or cardboard.