Irradiation of solutions of tetraphenylporphyrin (H2TPP) in chloroform causes decomposition of the chloroform at UV wavelengths higher than those that decompose chloroform directly. The catalytic cycle involves photooxidation of the porphyrin followed by thermal reduction. Photocatalysis continues after H2TPP has been completely protonated by the HCl produced during decomposition, and the rate of HCl production accelerates as a second pathway, in which CCl3OOH oxidizes the porphyrin, becomes important.