The present invention relates to a process for modifying an aqueous polymer latex of polymer particles that bear carboxyl groups comprising the mixing of the aqueous polymer latex with at least one silane compound of formula (1) at a temperature of at least 60° C.: (RO)3-nR1nSiR2X (1) wherein R is C1-C10-alkyl, C3-C10-cycloalkyl, C6-C10-aryl, C1-C10-alkoxy-C1-C10-alkyl, C2-C10-alkenyl, C1-C4-alkyl-C6-C10-aryl or C6-C10-aryl-C1-C4-alkyl, R is C1-C10-alkyl, C3-C10-cycloalkyl, C6-C10-aryl, C2-C10- alkenyl, C1-C4-alkyl-C6-C10-aryl or C6-C10-aryl-C1-C4-alkyl, R2 is a covalent chemical bond or a C1-C20-alkylene, in which non-adjacent methylene units can be replaced by O-groups, n is 0, 1, 2 or 3, and X is a radical having 3 to 10 carbon atoms and bearing an oxirane group. The present invention also relates to modified aqueous polymer latexes obtainable by such process and to the use of these polymer latexes as binders or co-binders for industrial varnishes.