The mechanism of spectral sensitization of SnO2 electrodes by ocladecyl-substituted cyanine dyes has been investigated. The Langmuir-Blodgett technique was used to manipulate the concentration of the adsorbed dyes and to vary the distance over which the sensitization occurs. When bis[1-octadecylbenzthiazol-2-yl]monomethinecyanine perchlorate THIAM 18 was excited at 435 nm, the distance dependence of the fluorescence quenching and of the anodic photocurrent could not be explained by the conventional mechanism of direct charge injection. In addition, a targe cathodic photocurrent was obtained at electrode potentials cathodic of 0.15 +/- 0.02 V versus the SCE. When THIAM 18 was replaced by the bis[1-octadecylbenzoxazol-2-yl]trimethinecyanine perchlorate OXA18, the anodic photocurrent could still be obtained but the cathodic photocurrent was reduced by at least 1 order of magnitude. A detailed study of these systems showed that, under favorable circumstances, energy transfer can be the main path to photosensitization.