A condensation polymer, obtainable by condensation of a mixture containing, as chief components,(A) from 90 to 99.9% molar of a mixture substantially consisting of(a) from 30 to 99% molar of melamine and(b) from 1 to 70% molar of a substituted melamine of the general formula I ##STR1## in which X, X', and X" are selected from the group consisting of --NH.sub.2, --NHR, and --NRR', and only one or two of X, X', and X" denote(s) --NH.sub.2, and R and R' are selected from the group consisting of hydroxy-C.sub.2 -C.sub.10 -alkyl, hydroxy-C.sub.2 -C.sub.4 -alkyl-(oxa-C.sub.2 -C.sub.4 -alkyl).sub.n, where n is an integer from 1 to 5, and amino-C.sub.2 -C.sub.12 -alkyl, or mixtures of melamines I, and(B) from 0.1 to 10% molar, based on (A) and (B), of unsubstituted phenols or phenols substituted by radicals selected from the group consisting of C.sub.1 -C.sub.9 -alkyl and hydroxy, or of C.sub.1 -C.sub.4 -alkanes substituted by two or three phenol groups, di(hydroxyphenyl)sulfones, or mixtures of said phenols,with formaldehyde or formaldehyde-liberating compounds, the molar ratio of melamine to formaldehyde ranging from 1:1.15 to 1:4.5.