The kinetics of the hydride transfer reaction between leuco methylene blue and thionine was investigated spectrophotometrically by means of the stopped-flow technique. Leuco methylene blue and leuco thionine were produced by photoreduction of methylene blue and thionine with triethylamine in ethanol in the drive syringe of the stopped-flow apparatus and their solutions were used as such in the measurement of the reaction rate. This reaction was found to be reversible, and the rate constants for forward and backward reactions were obtained. The ratio of k_f/k_b is consistent with the value of the equilibrium constant (K) which was obtained from the absorbance at 655 nm of methylene blue at equilibrium using the stoichiometric relationships and conservation conditions. The mechanism of this reversible hydride transfer reaction was discussed.