The kinetics at constant area of the monolayer reaction of chlorophyll a to pheophytin a on an acidic substrate were studied. The rate constants were found to depend upon pH, the initial surface pressure, the presence of O2 at the interface, and divalent metal ions such as Ca+2 and Mg+2 in the subphase. The half-life of chlorophyll a at 23° on a pH 4 substrate and at an initial surface pressure of 6 dynes/cm. was about 6 min. Evidence was obtained for the participation of H2O in the reaction and the probable existence of a charged intermediate.