The stability constant,, for the reaction of molybdenum(VI) with ethylenediaminediacetic acid (EDDA) and protonation constants were determined at T = 298 K and constant ionic strength of 0.1 mol.dm(-3) sodium perchlorate in five different compositions of water + methanol by a combination of potentiometric and UV spectrophotometric methods. Stability and protonation constant variations versus the composition of different water + methanol mixtures have been reported, and their values have been calculated on the basis of die continuous variation method.