The mass spectra of the following olefin derivatives of metal carbonyls have been investigated: (a) Arene—tungsten tricarbonyl complexes of toluene, p-xylene, and mesitylene; (b) The cycloheptatriene—metal tricarbonyl complexes C7H8M(CO)3 (M = Cr and W); (c) The 1,3,5-cyclooctatriene complex C8H10W(CO)3; (d) The cyclooctatetraene—metal tricarbonyl complexes C8H8M (CO)3 (M = Cr or Fe); (e) The 1,5-cyclooctadiene—metal tetracarbonyl complexes C8H12M(CO)4 (M = Mo or W); (f) The tungsten tetracarbonyl complexes of bicyclo[2,2,1] heptadiene and dicyclopentadiene; (g) The 1,3-cyclohexadiene complexes (C6H8)2M(CO)2 (M = Mo or W); (h) The cycloheptadienol complex (C7H9OH)Fe(CO)3; (i) The methyl vinyl ketone complexes (CH3COCH = CH2)3M (M = Mo or W); (j) A product of composition C12H16Fe(CO)2 obtained from allene dimer and Fe3(CO)12. The following fragmentation processes of particular interest were observed: (a) The facile dehydrogenation of ions containing the π-bonded 1,3,5-cyclooctatriene, 1,5-cyclooctadiene, and 1,3-cyclohexadiene ligands with two adjacent CH2 groups; (b) Elimination of neutral C5H6 fragments from tungsten carbonyl ions containing π-bonded bicyclo[2,2,1] heptadiene or dicyclopentadiene ligands; (c) Facile dehydration of iron carbonyl ions with π-bonded cycloheptadienol.