Theory of a 4 pH point acid titration method is presented to measure the H 2 CO 3 * alkalinity and the total carbonate species (C T ) in aqueous solutions containing only the carbonate weak acid/base. The influence of a systematic pH measurement error (due to faulty calibration, residual liquid junction effect, temperature) on the calculation of C T and H 2 CO 3 * alkalinity is examined, and methods presented to minimise the influence of the error. The influence of CO 2 loss during titration on C T and H 2 CO 3 * alkalinity estimates is shown to be negligible provided the titration is completed within 10 min with gentle stirring. Comparative tests using the 4 pH point titration and First Gran Function methods, on aqueous NaHCO 3 solutions ranging from 10 to 50 mg/l as CaCO 3 gave closely equal results with r = 0,99