We report on the pyrolytic metalorganic vapour phase epitaxy of ZnSe, ZnS and ZnSSe by using diethyldisulphide [(C 2 H 5 ) 2 S 2 ] and dimethyldiselenide [(CH 3 ) 2 Se 2 ], these dialkyls being suitable for the low (<400 o C) temperature growth of S- and Se-based compounds. (C 2 H 5 ) 2 S 2 and (CH 3 ) 2 Se 2 allow a substantial reduction of ZnSe and ZnS growth temperatures with respect to diethylsulphide and dimethylselenide. Mass spectrometry (MS) fragmentation products of (C 2 H 5 ) 2 S 2 and (CH 3 ) 2 Se 2 molecular ions are studied to investigate the relative strengths of specific bonds in the molecules and to identify their decomposition paths. The decomposition of (C 2 H 5 ) 2 S 2 occurs mainly via the loss of ethylene molecules through β-hydrogen elimination reactions. On the contrary, the sequential loss of methyl radicals seems the dominant path of (CH 3 ) 2 Se 2 , β-like hydrogen elimination reactions being still possible, although less likely than for (C 2 H 5 ) 2 S 2 . The occurrence of a peculiar CH 3 transposition reaction is also suggested for (CH 3 ) 2 Se 2 . Weak or negligible [for (C 2 H 5 ) 2 S 2 ] contributions to the alkyl mass spectra are observed from Se-Se or S-S bond cleavage. This is ascribed to the strength of the Se-Se (or S-S) bond in the (CH 3 ) 2 Se 2 [(C 2 H 5 ) 2 S 2 ] molecule, which would destabilise the Se-C (S-C) bonds, leading to the alkyl low thermal stability. Low H concentrations, i.e. (1-3)x10 1 7 cm - 3 , are found by secondary ion MS in ZnSe samples and attributed to the low proclivity of (CH 3 ) 2 Se 2 to decompose through β-like hydrogen reactions.