Measurement of spin and free halide concentrations in solutions of trans-[MoOL(MeOH)]Br (H2L = H2SALOPHEN, H2SALPN, HjSALEN) and [MoO(SALPN)Cl] in DMF (1-5 mM Mo; 0.1 M Et4NX; X = PF6, Cl, Br) indicates the presence of equilibria between mononuclear and polynuclear species. For [MoO(SALOPHEN)(MeOH)]Br in the presence of 0.1 M PF6-, the mononuclear cation trans-[MoO(SALOPHEN)(DMF)]+ is present at a concentration of about 100 mol % but equivalent solutions of [MoOL(MeOH)]Br (H2L = H2SALPN, H2SALEN) contain low concentrations (ca. 10 mol %) of ESR-silent polynuclear species containing bound bromide. For (MoO(SALPN)Cl] solutions, the concentration of polynuclear species is much higher, being >50 mol % Mo in PF6- media and >85 mol% in Cl- media. The dominant mononuclear species appears to be [MoO(SALPN)Cl]. The electrochemistry of these systems is complicated by the equilibria and an unequivocal interpretation is not possible from the present (quite extensive) data. Examination of the first reduction process for each system suggests that the halide-bound complexes are electroactive, even for [MoO(SALOPHEN)(MeOH)]Br in 0.1 M PF6- media where the free bromide concentration is about 100 mol %. For [MoO(SALPN)Cl], reduction of both mononuclear and polynuclear species is observed. The total data appear to reflect the tendency of mononuclear oxomolybdenum(V) species (4d*) to associate in solution to form binuclear or polynuclear products that are stabilized by spin-spin interactions, whereas the oxomolybdenum(IV) analogues tend to thermodynamically favor mononuclear (low-spin d2) forms.