We report the first measurements of rate constants for formation and reaction of the hydrated-hydride ion H3O-. We studied the Kleingeld-Nibbering reaction [Int. J. Mass Spectrom. Ion Phys. 49, 311 (1983)], namely, dehydrogenation of formaldehyde by hydroxide to form hydrated-hydride ion and carbon monoxide. The OD- +H2CO reaction is about 35% efficient at 298 K, with OD-/OH- exchange occurring in about half the reactions. H3O- was observed to undergo thermal dissociation in a helium carrier gas at room temperature with a rate constant of 1.6 x 10(-12) cm3 s-1. We also studied a new reaction in which H3O- is formed : The association of OH- with H-2 in a He carrier gas at low temperatures. The rate coefficient for this ternary reaction is 1 X 10(-30) cm6 s-1 at 88 K. Rate coefficients and product branching fractions were determined for H3O- reactions with 19 neutral species at low temperatures (88-194 K) in an H-2 carrier. The results of ion-beam studies, negative-ion photoelectron spectroscopy, and ion-molecule reaction data allow us to specify the hydride-water bond energy D(298)0(H--H2O) = 14.4 +/- 1.0 kcal mol-1 (0.62 +/- 0.04 eV). The heat of formation of H3O-, -37.5 +/- 1.0 kcal mol-1, and the proton affinity of H3O-, 386.0 +/- 1.0 kcal mol-1, are derived from these results. Dissociation of H3O- into OH- and H2 requires 4.5 +/- 1.0 kcal mol-1 energy.