The invention relates to aqueous crosslinking polymer dispersions which are characterized in that they consist ofA) 98 to 80 wt. % of a 40 to 60% aqueous alkaline polymer dispersion based on 2-chlorobuta-1,3-diene, with a pH of .gtoreq.9.5 and a chloride content, arising from the elimination of chlorine atoms from the polymer, of .gtoreq.300 ppm, andB) 2 to 20 wt. % of polyisocyanates obtained by the cyclotrimerization of aliphatic and/or cycloaliphatic diisocyanates, containing NCO groups bonded predominantly or exclusively as secondary groups,and to their use for the preparation of water-based adhesives.