A simple means to differentiate these scenarios is to compare the kinetics of release of PB + OH - as a reference with that of PBMe + HCO3 -. The rate of mechanism 1, linear in hydroxide concentration, should be several orders of magnitude slower with CO2 exposure than without, as the pH of the solution decreases by several units. Using in situ UV-vis spectroscopy, we followed the decay of the carbocation in a ca. 10 mM solution of PBMeOH , prepared by photolysis, with and without pre-exposure to CO2 (1 atm) (Figure plot). Both time courses fit well to second-order rate equations with observed rate constants of 7.01 mM -1 s -1 without CO2 and 1.36 mM -1 s -1 after CO2 exposure. The fact that the latter profile fits a second-order apparent rate law is evidence against mechanism 1, as the hydroxide concentration would not be directly proportional to the carbocation concentration with CO2 present.