The apparent proton dissociation constants of a commercial alginic acid have been obtained in KNO3 and NaCl at concentrations ranging between 0.01 and 2 mol L-1. An analysis of the dependence on the ionic strength at a constant value of the degree of dissociation was done by means of empirical functions derived from the Gibbs-Donnan formalism for polyelectrolytes and a specific-ion interaction theory (SIT). Both functions were able to fit the experimental data, although SIT yielded rather high errors in the fitted parameters due to a problem of multicollinearity, in contrast to the function derived from the Gibbs-Donnan approach. (C) 2003 Published by Elsevier B.V.