Ten new nonsymmetric dithiolenes of the general formula [RR’(C2S2)](3)M were synthesized, from the corresponding 1,3-dithiol-2-ones, or from the corresponding mixed benzoins and tetraphosphorus decasulfide. The redox properties of these ten complexes together with those of the symmetric ones with R = R’ = Ph and M = Mo or W, were investigated by cyclic voltammetry. Two processes were only observed, assigned to C --> C1- and C1- --> C2- (C the complex), for which the quasi-reversible E(1/2) potentials are highly dependent on the nature of the central metal ion, and for the same metal on the nature of the ligand and on the substituents on the phenyl rings. The ease by which the complexes are reduced is most likely the reason they give ESMS (negative mode) spectra, in spite of the fact they are neutral in solution. The process of this reduction and the parameters affecting it are investigated and discussed.