Monolayers of n-alkanethiols of chain length from C-12 to C-18 were self-assembled on a hanging mercury drop electrode, and a film of chlorophyllide (Chlide) was adsorbed on top of them. The reduction photocurrents following illumination of the Chlide film were measured over the potential range in which the Chlide is electroinactive in the dark, and their action spectra were determined. Plotting the derivative of the photocurrents with respect to the applied potential against potential yields bell-shaped curves that can be fitted to a Gaussian. The potential of the Gaussian maximum was used to determine the reorganization energy for the Chlide electroreduction process. An increase in the thiol chain length causes A to decrease regularly and the photocurrent to decay exponentially with the monolayer thickness, with a decay constant beta of about 0. 17 Angstrom(-1).