The equilibria [MoOCl3L2]⇌[MoOCl3L]+ L in CH2Cl2 solution at 25 °C have been studied by e.s.r. Spectroscopy. When L = Ph3PO, (Me2N)3PO, or thf, no [MoOCl3L] species have been detected, where as when L = Ph3PS, (Me2N)2CS, or Me2S, [MoOCl3L] predominates. The new complexes [MoOCl3L][L = Ph3PS, (Me2N)2CS, Ph3PSe, orPh3PO] and [MoOCl3{SC(Nme2)2}2] have been isolated. Similarly the equilibria [MoOCl4]–+ Ln–⇌[MoOCl4L](n+1)– have been studied [L = Ph3PO, (Me2N)PO, py, Et3N, thf, H2O, Ph3PS, and (Me2N)2CS, n= 0; L = Cl–, n= 1]. When L =(Me2N)3PO only [MoOC4L]– has been detected. With the other neutral donors the order of ability to form [MoOCl4L]– is py > Ph3PO ≈ H2O thf Et3N, whereas no six-co-ordinate species have been detected for L = Ph3PS or (Me2N)2CS. With L = Cl– the degree of formation of [MoOCl5]2–is low and dependent on both the nature of the cation and total molybdenum concentration. By comparison of the results in CH2Cl2, it is concluded that the molybdenum(V) species formed on dissolving [Et3N]2[MoOCl5] or MoOCl5 in concentrated aqueous HCl is [MoOCl4(OH2)]– and not [MoOCl5]2– as is generally supposed.