An additive breakdown product in an acid copper plating bath sample is detected by performing a voltammetric analysis for the leveler additive in two measurement solutions comprising different volume fractions of the copper plating bath sample in a background electrolyte. When the plating bath sample contains a substantial concentration of the additive breakdown product, the analyses for the first and second measurement solutions indicate different concentrations for the leveler additive in the plating bath sample. A comparison of the leveler additive concentrations indicated by the two analyses provides a measure of the concentration of the additive breakdown product.