The formation constants of the species formed in the systems H + + Mo(VI) + nitrilotriacetic acid and H + + nitrilotriacetic acid have been determined in different aqueous solutions of methanol at 25 -C and constant ionic strength 0.1 mol dm Γ 3 sodium perchlorate, using spectrophotometric and potentiometric techniques. The composition of the complex was determined by the continuous variations method. It was shown that molybdenum(VI) forms a mononuclear 1:1 complex with nitrilotriacetic acid of the type MoO 3 L Γ 3 at Γ log[H + ] = 5.8. The formation constant in various media was analyzed in terms of Kamlet and Taft's parameters. Single-parameter correlations of the formation constant, K S , versus a (hydrogen-bond donor acidity) and p* (dipolarity/polarizability) are poor in all solutions, except for b (hydrogen-bond acceptor basicity), but multi-parameter correlations represent significant improvement with regard to the single-parameter models. Linear relationship is observed when the experimental log K S is plotted versus the calculated ones, while all the Kamlet and Taft's parameters are considered. Finally, the results are discussed in terms of the effect of solvent on complexation.