An aqueous acid plating bath for the electro-deposition of a zinc or zinc alloy deposit on a substrate which includes zinc ions and an additive of the general formula ##STR1## wherein R.sub.1 and R.sub.2 are selected from the following functional groups hydroxyl, carbonyl, carboxylic acid or its salt; R.sub.3 is selected from the following groups hydrogen, hydroxyl, carbonyl, carboxylic acid or its salts; R.sub.4 and R.sub.5 are electron withdrawing groups, such as but not totally inclusive, halides, sulfonic acid or its salts, triflouromethyl, cyano, and amino groups.