According to its absorption spectra, chlorophyll a (chla) exists as (chla)2 and (chla . H2O)2 in aqueous acetone solutions (pH 6.8). These two states can be adsorbed on an SnO2 electrode and form a multilayer under conditions of saturated adsorption. An anodic peak at +0.23 V vs. Hg/Hg2SO4 in 0.1 M Na2SO4, with no corresponding cathodic peak, is observed on the cyclic voltammogram of the chla/SnO2 electrode. We ascribe this peak to a change in the capacity of the chla/SnO2 electrode.