Carboxylic groups in carboxylated butadiene-acrylonitrile rubbers (XNBR) may be sites of various crosslinsing reactions, the most important being formation of ionic crosslinks which, in combination with covalent crosslinks, ameliorate the physical properties, Additionally, due to the polar character of carboxylic groups, the surface activity of XNBR is augmented compared to that of ordinary NBR, thus intensifying rubber interactions with fillers. This is particularly important for compounds with non-black fillers which are in general less compatible with rubbers than carbon black. XNBR and silica form both covalent and ionic linkages, making the properties similar to those of corresponding networks with carbon black. Less active non-black fillers form weaker linkages with XNBR which, despite decaying, e.g. in the network swelling process in solvents, still act reinforcing under normal conditions.