Facile reduction of the triangular {Mo₃^IV}⁶-core containing [Mo₃S₄ (CN)₉]⁵⁻ derivative is achieved in aqueous medium, containing the KOH-KCN-K₂ [Zn(CN)₄] ternary system as supporting electrolyte, in a two-step one-electron reduction reaction followed by an irreversible one. The diamagnetic species containing the {Mo₃}⁸ core, obtained by chemical reduction of [Mo₃S₄(CN)₉]⁵⁻ with zinc in aqueous medium containing KOH-KCN, is oxidized by air in two steps. The partially oxidized species with {Mo₃}⁷ core in the first step exhibits characteristic ESR signals which on further oxidation quantitatively reverts to the original compound with {Mo₃}⁶ core displaying distinctly separate isosbestic points in the stepwise conversion of {Mo₃}⁸ to {Mo₃}⁷ and {Mo₃}⁷ to {Mo₃}⁶ cores. Under drastic reduction conditions with aluminium in 40% aqueous KOH solution the {Mo₃}⁶ core is reduced to unstable trinuclear {Mo₃^III}⁹ core which expands to tetrahedral {Mo₄^III}¹²-core containing [Mo₄S₄(CN)₁₂]⁸⁻ derivative according to electron-precise cluster rule.