ABSTRACT The interaction between a polyion and its counterion has been studied for a series of potassium salts of poly(maleic acid-co-vinyl-n-alkyl), PA-nK2, with n=8,10,12, 14 and 16 carbon atoms in the side chain. The polyion-counterion interaction parameter was determined from electrical conductivity measurement. This parameter is a function of the side chain length, and its variation is ascribed to a change in the average distance between charges