Protonation constants of succinic, 1,2,3-propanetricarboxylic and 1,2,3,4-butanetetracarboxylic anions were determined in NaCl(aq)+KCl(aq) mixtures, at three ionic strengths, I=1.2, 3 and 4.5molL(-1). Experimental evidences showed that the function log K(H)=f(y) (y=[Na(+)]/([Na(+)]+[K(+)])) is not linear, indicating mixing effects on the protonation constants. The Guggenheim zeroth approximation holds that the above function can be written as:where K(Na)(H)andK(K)(H) represent protonation constants in pure salt solutions and Delta is a parameter that accounts for the mixing effect. Fitting of protonation constants to the above function gives excellent results. The Delta values can be treated in terms of mixing free energy. The behaviour of protonation constants in mixed salt solution can be interpreted by considering the formation of simple and mixed weak complexes; the protonation constants in mixed NaKCl electrolytes can be fitted to the equation: log(10)K(Na-K)(H)=log(10)K(K)(H)-log(10)(1+A(1)C(Na)+A(2)C(Na)C(K)), where A(1) is a measure of the interaction of Na(+) with the carboxylic anion and A(2) is proportional to the triple interaction Na(+)-K(+)-L(z-). Moreover, by using suitable calculation methods, it is possible to calculate the formation constants of simple and mixed ion pairs. As an example, for 1,2,3,4-butanetracarboxylic anion (L(4-)), we calculated K(Na(+)+H(i)L((4-i)-)=NaH(i)L((3-i)-))=0.67, 0.33 and 0.13; K(K(+)+NaH(i)L((3-i)-)=KNaH(i)L((2-i)-))=1.41, 1.29 and 0.9 for i=0, 1 and 2, respectively, indicating a significant tendency to form mixed alkali metal ion pairs.