The reaction of photogenerated singlet oxygen with α- and β-pinenes has been carefully re-examined. α-Pinene almost exclusively (99.3% yield) furnishes trans-3-hydroperoxy-pin2(10)-ene. However, detectable amounts of the three other possible products are also found, viz., cis-3-hydroperoxypin-2 (10)-ene (˜0.8%), and cis- and trans-2-hydroperoxypin-3 (4)-ene (˜0.04%). β-Pinene gives 99.9% of 10-hydroperoxypin-2 (3)-ene and a trace (˜0.01%) of norpinan-2-one. Rates of reaction and product composition are treated by modal analysis and reflect the operation of steric and stereoelectronic factors in a reactant-like transition state.