The solution equilibrium of the molybdenum(VI) complex formed by ethylenediaminediacetic acid (EDDA) has been investigated potentiometrically and spectrophotometrically at 25 degrees C, pH 6.0, and different ionic strengths ranging from (0.1 to 1.0) mol dm(-3) of sodium perchlorate. Molybdenum(VI) forms a mononuclear complex with EDDA of type (MoO(3)L(2-)). The dependence of dissociation and stability constants on the ionic strength is described by a modified Debye-Huckel equation and by introducing two empirical parameters. Finally, a comparison has been made between the calculated and experimental values of stability constants.