Protonation of the µ₃-alkylidyne cluster [Os₃(µ-H)₂(CO)₉(µ₃-CNC₅H₄C₅H₄N)]1(NC₅H₄C₅H₄N = 4,4′-bipyridine) took place in two consecutive steps upon treatment with strong protonic acids such as HBF₄ or CF₃CO₂H in dichloromethane. Initial protonation occurs at the organic ligand to give quantitatively the monocationic dihydrido species [Os₃(µ-H)₂(CO)₉(µ₃-CNC₅H₄C₅H₄NH)]⁺2. In the presence of an excess of acid a second protonation takes place at the metal core and 2 is rapidly converted into a water-soluble dicationic trihydrido cluster [Os₃(µ-H)₃(CO)₉(µ₃-CNC₅H₄C₅H₄NH)]²⁺3. The crystal structure of the bis(tetrafluoroborate) salt as its monohydrate has been determined, and reveals the presence of extensive hydrogen bonding. The formation of complexes 1–3 is found to be reversible by successive protonation and deprotonation processes, and is reminiscent of typical acid–base equilibria. On the other hand, the reaction of 1 with [Me₃O]⁺BF₄^– in dichloromethane afforded the N-methylated derivative, [Os₃(µ-H)₂(CO)₉(µ₃-CNC₅H₄C₅H₄NMe)]⁺4, which gave another dicationic complex [Os₃(µ-H)₃(CO)₉(µ₃-CNC₅H₄C₅H₄NMe)]²⁺5 on protonation with HBF₄ or CF₃CO₂H. Similar to 3, complex 5 is very soluble in water.