Studies are presented which demonstrate that: (1) irradiation of titanium and zinc oxide pigments produces singlet oxygen; (2) irradiation of titanium dioxide pigments in water yields hydrogen peroxide; and (3) the formation of singlet oxygen and hydrogen peroxide correlates with chalking tendencies of pigments. These findings, together with the results of quenching studies, are interpreted in terms of a working hypothesis for the generation of reactive oxidants, which ties together previous work into a unified scheme. -- AATA