The kinetics of interaction of Mo2Oinf4sup2+ with VO2+ was studied in HC1O4 medium at 25° C and ionic strength of 2 M by the stopped-flow technique. Under conditions of excess VO2+ a reaction mechanism is proposed to account for the observed pseudo-first order kinetics. For the overall process, 1/2 [Mo2O4(H2O)6]2+ + VO2+⇋C + H+, the equilibrium constant (K = 33 M1/2) was determined by absorbance measurements.