Anodic oxidation of dimethyl sulfoxide (DMSO) based electrolyte solutions, containing LiClO4, LiBF4 and KPF6, on platinum (Pt), glassy carbon (GC) and n-TiO2 (anatase), electrodes was studied using in situ Fourier transform infrared spectroscopy (FTIR). All solutions contained small amounts of H2O. Regardless of the supporting electrolyte all systems were unstable at potentials above 1.0 V vs SCE. The major oxidation product is dimethyl sulfone, formation of which is initiated by the trace water breakdown. In contrast to acetonitrile based solutions there is no evidence of electrolyte involvement in the breakdown process. Photoanodic decomposition of dimethyl sulfoxide based solutions proceeds in the same way as the anodic oxidation in the dark. In the presence of nucleophilic agent (iodides) the prevailing redox process is iodide oxidation. Small amounts of, probably, methylsulfinyliodide are also formed. The irreversible consumption of charge mediator significantly restricts the possible practical use of DMSO in photoelectrochemical devices.