Photoinduced electron transfer from eleven metallochlorophyll a (MChl a) monolayers to a SnO 2 substrate was investigated photoelectrochemically. Six MChl as [M = Pd(II), Cu(II), Hg(II), H 2(II), Mg(II), and Zn(II)] gave measurable photoeffects. For the latter three pigments, two-dimensional dilution of the monolayer led to an enhancement of the photocurrent quantum yield. The highest quantum yield was attained with MgChl a at sufficient dilution. Five MChl as [M = Ni(II), Co(II), Ag(II), Mn(III), and Fe(III)] showed no photoresponse, due probably to a rapid intramolecular deactivation of the excited state. Some kinetic aspects of the interfacial photoprocess were clarified by experiments with mixed monolayers composed of a photoactive MChl a and photo-inactive NiChl a (quencher).