The ion-molecule reaction OH- + H2CO --> H3O- + CO has been studied at 300 K with isotopic labeling of reactants. The H3O- product is only observed in small abundance because the ion dissociates into OH- + H-2 upon multiple collisions in a helium buffer gas. Without isotopic labeling, the pseudo-first-order kinetics plots for the reactions of OH- with H2CO and OD-+ D2CO were found to be curved as a result of the regeneration of OH- or OD- reactant. A scavenger technique was used to remove the H3O- (or D3O-) produced prior to dissociation, to reveal the true first-order attenuation of OH- (or OD-) in reaction with H2CO (or D2CO). The rate constant for the OH- + H2CO reaction is 7.6 X 10(-10) cm3 s-1, and for OD- + D2CO is 5.7 X 10(-10) cm3 s-1. For the isotopically mixed cases OH - + D2CO and OD- + H2CO, the rate constants are equal to 1.3 X 10(-9) cm3 s-1, about twice as large as those for the reactions involving only a single hydrogen isotope, indicating that isotopic exchange is an important process. The rate constants for the thermal dissociation of H3O- and D3O- in helium were found to be 1.6 X 10(-12) and 1.1 X 10(-12) cm3 s-1, respectively, within a factor of 2. The results are discussed in terms of other thermal dissociation reactions of ions.