The full kinetic pH profile for the base-promoted decomposition of MTO to CH4 and ReO4- was examined with the inclusion of new data at pH 7-10. Spectroscopic and kinetic data gave evidence for mono- and dihydroxo complexes : MTO(OH-) and MTO(OH-)(2). Parallel unimolecular eliminations of methane from these species account for the rate-pH profile; the respective rate constants are MTO(OH-)(2), k = 4.56 x 10(-5) s(-1) and MTO(OH-)(2), k = 2.29 x 10(-4) s(-1) at 25 degrees C. Some kinetic data were acquired with electrospray mass spectrometry to monitor the buildup in the concentration of perrhenate ions. Reliable signals for each of the isotopomers of ReO4- were obtained by this method with initial MTO concentrations of 160 mu M.