The mass spectra of the polynuclear carbonyls MnRe(CO)₁₀, Fe₂(CO)₉, Os₃(CO)₁₂, Os₄O₄(CO)₁₂, Co₂(CO)₈, Co₄(CO)₁₂, and Rh₄(CO)₁₂ are reported and where possible interpreted in terms of their structures. In all cases the molecular ion peak was observed and the predominant fragmentation scheme involved the successive removal of the CO groups before rupture of the central nucleus.