Salts of organic optical brighteners or light stabilizers with compounds which contain at least one ammonium, cycloammonium or immonium group and at least one aliphatic group, cycloaliphatic group, or aromatic group bonded to at least one (up to four) carbonyl group, or heteroaromatic group. resp. basic brighteners or light stabilizers with compounds which contain a radical containing at least one carboxylic acid or sulphonic acid radical and one aliphatic, cycloaliphatic, monomeric aromatic or heteroaromatic group, or contain a C.sub.2-14 -dicarboxylic acid or amino-C.sub.2-1 -alkylcarboxylic acid radical, are eminently suitable for the brightening and light stabilizing of synthetic plastic compositions.