The thermally activated recombination reaction O+O-2-->O-3 is observed in solid xenon matrices and in free-standing crystals of xenon at 14-25 K on the time scale 10(2)-10(5) s. The reactants are prepared as spatially separated O...O-2 pairs immobilized in solid Xe at 10 K by 266 nm photodissociation of O-3 precursor molecules. The temperature dependence of the ozone recovery rate yields an activation energy for diffusion of O atoms in solid xenon of 2.0+/-0.5 kJ/mol. This value also represents an upper limit to the potential energy barrier to the O+O-2 recombination reaction itself. In dilute samples (mole fraction of ozone less than 2X10(-4)) more than 90% of the initial O-3 is recovered during the warming cycle. Only a small fraction of O atoms (<20%) escape geminate recombination with the partner oxygen molecule within the pair and react with other O, molecules or O atoms. The experimental results are interpreted within the framework of a continuous diffusion model in which the initial spatial distribution of reactants is nonrandom.