The electrochemical behaviour of 4,6-dimethyl-2-thiopyrimidine (LH) and 4,6-dimethyl-1-phenyl-2-thiopyrimidine (LPHI) and their protonated forms was investigated in dimethylformamide on Hg electrodes. Adsorption processes affect the oxidation mechanism of both the compounds and the formation of Hg(I) complexes was observed. The electrochemical reduction of LH provides an example of the so-called "father-son reaction" but this kind of mechanism cannot occur with LPHI. Only in the species LH is a proton present which can be abstracted by the product of the first electron transfer LH.- while LPHI can be protonated by the solvent. However, in the presence of strong acids, for both molecules LH and LPHI the formation of the dimer from the radical anion obtained in the first electron transfer is kinetically preferred to the father-son reaction.