The compounds M(Se 2 CNEt 2 ) 2 , M = Zn 1 or Cd 2, M(Se 2 CNMe n Hex) 2 , M = Zn 3 or Cd 4, and ZnEt(Se 2 CNEt 2 ) 5 have been investigated by GC-MS and EI-MS in an attempt to determine why 1 and 2 often deposit films containing an excess of selenium by metal organic chemical vapour deposition (MOCVD) whereas 3, 4 and 5 give MSe (M = Zn or Cd). The EI-MS of all five selenium compounds revealed similar decomposition pathways, which start with loss of an alkyl group. However, studies of compounds 1–5 by pyrolysis GC-MS showed that selenium clusters Se n , n = 1–7, are formed in the vapour phase from 1 and 2. Compounds 1, 2 and 5 also form significant quantities of diethyl diselenide (EtSe 2 Et) in the ratio of 1∶0.9∶0.2 respectively, as detected by GC-MS, whereas 3 and 4 do not.