The kinetics of the equilibration of molybdenum (V,V) complexes of Mo2O42+ (A) and Mo2O4Y2- in the presence of excess of EDTA (H4Y = ethylenediaminetetraacetic acid) have been studied by spectrophotometry methods at 25 degrees C, and ionic strength, I=1.50M (LiClO4). First order rate constants at constant hydrogen-ion concentrations in the range [H+] = 0.2-1.8M may be expressed as k(aq) = a[EDTA]+b. The hydrogen dependences of a and b, corresponding to the formation and dissociation processes respectively are not simple For the dissociation process the equation b = [H+](4)/(1+b(2)[H+](3)) gives a good fit the experimental data. Contributions from terms in [H+] and [H+](2) in the denominator can not be ruled out however. An inerse hydrogen-ion dependence is apparent for a but the precise form is less certain. The substitutional and redox properties of mono- and binuclear complexes of molybdenum with different ligands N, O and S atoms are of interest as models for molybdenum enzymes [1]. It has been now established that most of these enzymes contains two molybdenum atoms which are redox active. The molybdenum are in oxidation state (VI) in the oxidized form, and can utilize other oxidation states, in some cases down to (III) in the reduced form [2].