Doping of ZnO samples with Li** plus and Al**3** plus ions affects the photocatalytic oxidation of isopropanol on the illuminated sample. At low concentrations Al**3** plus increases the yield of thye photoproduct, viz. peroxide, while Li** plus suppresses it. While the peroxide yield reaches a limiting value as the amount of Li** plus is increased, with increasing amounts of Al**3** plus the peroxide yield reaches a maximum value and drops to a value even lower than for an undoped oxide. This anomalous behavior of excess Al**3** plus is examined.