AbstractCounterion activity of poly‐S‐carboxyethyl‐L‐cysteine in salt‐free solutions neutralized to various degrees with sodium hydroxide was determined either directly from the e.m.f measurements of concentration cells formed across a Na–glass electrode, or from the potentiometric titrations at different sodium chloride concentrations assuming the additivity rule.From e.m.f. measurements, the activity coefficient of counterions was confirmed to display the β‐structure random coil transition of the polymer.For random coil state, both methods gave identical results. Observed values of the activity coefficient was generally smaller than those for other randomly coiled polypeptides. The activity coefficient was found to decrease with the increase of the polymer concentration.The activity coefficient of counterions for β‐structure was extremely small as compared with that for random coil at the same degree of neutralization. The activity coefficient obtained from titrations was almost independent of degree of neutralization and increased with the increase of the poly