Electrohydrodimerization (EHD) of ethylcinnamate (EC) on mercury takes place both from hydrotropic solutions of tetraethylammonium-p-toluenesulphonate (TEA-PTS) and from dilute aqueous solutions of the strong surfactant Triton X-100. In both cases the one-electron reduction wave due to hydrodimer formation satisfies the requirements for a rate-determining homogeneous coupling of the electrochemically generated anion radicals. The kinetics of EC EHD on liquid gallium is identical with that on mercury at the same temperature of 31 degrees C in hydrotropic solutions of TEA-PTS; conversely, in aqueous Triton X-100 it is faster than on mercury and satisfies the requirements for a rate-determining heterogeneous radical-radical coupling. This behaviour is explained by a stronger adsorptivity of EC and its intermediate reduction products on gallium than on mercury, so that Triton X-100 does not succeed in displacing these species completely from the electrode surface. The general trend consisting in a higher adsorptivity of organic compounds with conjugated double bonds and/or aromatic rings on gallium than on mercury is explained on the basis of the difference in hydrophilicity between these two metals.