The EI-mass spectra of the dimeric η3-allylnickel(II) halides (C3H5NiX)2 (X = Cl, Br, I) were recorded. Besides the successive splitting-off of the C3H5 groups and the elimination of C3H5X, the formation of NiX2 leading to (C3H5)2Ni is the predominating fragmentation path. Cleavage of the dimeric structure is observed only in the case where X = I.