The electrochemistry of dioxygen reduction in the presence of a model molecule of biological significance, 4,6-dimethyl-2-thiopyrimidine (LH) was studied in dimethylformamide at a glassy carbon (GC) electrode. The first, mono-electronic, reduction step took place under kinetic control of charge transfer or of the following chemical reactions, or both, depending on the time scale of the experiment, leading to a complicated pattern of events. A father-son reaction step and other cross protonation reactions could explain the detailed results obtained from the electrochemical investigation. The involvement of LH in the dioxygen reduction path was indicated by the presence of a new cathodic peak in cyclic voltammetry, which did not pertain to the voltammetric behavior of either species, and was shifted by ca. 70 mV from that due to the O-2/O-2(.-) reduction in the absence of LH. The experimental results could be consistently interpreted by assigning the new peak to the O-2/O-2(.-) coupled with a fast protonation of O-2(.-) by LH, and explaining the amount of the displacement by the rate of the protonation reaction.