The radical ions (SCN)(2)(.-) and I-2(.-) were produced by photocatalytic flash excitation (355 nm) of oxygenated TiO2 sols suspended in 1.00 M aqueous solutions of KSCN or KI. To determine the effect of surface heterogeneity of the photocatalyst on the secondary reactions of these transients, these radical ions were reacted with a series of electron donors. The resulting heterogeneous rate constants are compared to those known for the same reactions in homogeneous solution, where (SCN)2(.-) and I-2(.-) were generated by pulse radiolysis. Relative to the solution phase, the reaction between (SCN)2(.-) and ascorbic acid on a TiO2 sol was accelerated by approximately a factor of 40, that between (SCN)(2)(.-) and hydroquinone was essentially unchanged, and that between (SCN)2(.-) and cysteine was slowed by approximately an order of magnitude. The dismutation of(SCN)(2)(.-) on TiO2 was accelerated by a factor of 3. The reaction of I-2(.-) with cysteine on a TiO2 sol was approximately 25 times slower than that in solution, and its dismutation was accelerated by a factor of 4. These changes in kinetic selectivity are explained by the association of the photocatalytically generated radical ion with the TiO2 surface.