A range of 19 different 5-phenyl-1,2-dithiole-3-thiones, meta-or para-substituted, were investigated in a DMF electrolyte at a platinum electrode. In all cases, excepted for the para-nitrophenyl derivative, the reduction appeared as a quasi-reversible process at 90 degreesC while it was generally irreversible at 20 degreesC. The redox potential of the quasi-reversible reduction (E-redox) or the peak potential of the irreversible reduction (E-pc) followed a linear correlation involving the Hammett parameters sigma(m) and sigma(p) related to the phenyl group substituent. Good linear Correlation relationships were obtained regardless of the characteristic potential (E-pc or E-redox) or the temperature: at 20 degreesC E-pc(x) (V/Fc(+)/Fc) = 0.20sigma(x)-1.54 (r = 0.96) and at 90 degreesC E-redox(x) (V/Fc(+)/Fc) =0.16sigma(x)-1.46 (r =0.97). In the case of strong electron-withdrawing substituents (para-CN or para-CO2CH3) the compounds showed quasi-reversible behaviour even at room temperature.