This paper discribes measurements of the sensitized charge transfer efficiency of dimers compared to monomers in a rigid monolayer structure of cyanine dye molecules on the surface of p-chloranil single crystals. Relations for monomer/dimer ratios are derived from absorption spectra and action spectra of the photocurrent. Comparison yields a minor but comparable injection probability for dimers with respect to monomers. The results are discussed in terms of energy relations, separation of reaction sites and steric arrangements and are compared with those for dyes in solution, the sensitizing efficiency of which is governed by their adsorption kinetics rather than by their electrochemical photoreactivity at the electrode. From competing quenching measurements at an adjacent metal electrode a rate constant for the sensitized charge transfer, kCT≈ 1010 s–1, is estimated.