The stable compound CH3ReO3 (MTO), upon treatment with aqueous hypophosphorous acid, forms a colorless metastable species designated MDO, CH3ReO2(H2O)(n) (n = 2). After standing, MDO is first converted to a yellow dimer (lambda(max) = 348 nm; epsilon = 1.3 x 10(4) L mol(-1) cm(-1)). That reaction follows second-order kinetics with k = 1.4 L mol(-1) s(-1) in 0.1 M aq trifluoromethane sulfonic acid at 298 K. Kinetics studies as functions of temperature Save Delta S-not equal = -4 +/- 15 J K-1 mol(-1) and Delta H-not equal = 71.0 +/- 4.6 M mol(-1). A much more negative value of Delta S-not equal would be expected for simple dimerization, suggesting the release of one or more molecules of water in forming the transition state. If solutions of the dimer are left for a longer period, an intense blue color results, followed by precipitation of a compound that does, even after a long time, retain the Re-CH3 bond in that aq. hydrogen peroxide generates the independently known CH3Re(O)(O-2)(2)(H2O). The blue compound may be analogous to the intensely colored purple cation [(CP*Re)(3)(mu(2)-O)(3)(mu(3)-O)(3)ReO3](+) If a pyridine N-oxide is added to the solution of the dimer, it is rapidly but not instantaneously lost at the same time that a catalytic cycle, separately monitored by NMR, converts the bulk of the PyO to Py according to this stoichiometric equation in which MDO is the active intermediate: C5H5NO + H3PO2 --> C5H5N + H3PO3 A thorough kinetic study and the analysis by mathematical and numerical simulations show that the key step is the conversion of the dimer D into a related species D* (presumably one of the two mu-oxo bonds has been broken); the rate constant is 5.6 x 10(-3) s(-1). D*; then reacts with PyO just as rapidly as MDO does. This scheme is able to account for the kinetics and other results.