A composition comprises a carboxylic acid compound which contains at least three carbon atoms together with an organic sulphur-containing acid or salt of such an acid. The carboxylic acid may be an aliphatic diacid such as succinic acid. The sulphur-containing acid may be a sulphonic acid or sulphuric acid derivative such as the sodium salt of an alkyl sulphonic acid or of sulpho-succinic acid or of a carboxylic acid ester derivative of sulpho-succinic acid. The composition may be incorporated into an olefin polymer. The composition may be obtained by conventional powder blending or, for a polymer composition, melt blending techniques. Preferred polymer compositions have an increased clarity and a reduced level of haze compared to the polymer alone or the polymer containing only the carboxylic acid.