We report on the pyrolytic metalorganic vapour phase epitaxy of ZnSe, ZnS and ZnSSe by using diethyldisulphide [(C2H5)(2)S-2] and dimethyldiselenide [(CH3)(2)Se-2], these dialkyls being suitable for the low (<400 degrees C) temperature growth of S- and Se-based compounds. (C2H5)(2)S-2 and (CH3)(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 (C2H5)(2)S-2 and (CH3)(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 (C2H5)(2)S-2 occurs mainly via the loss of ethylene molecules through beta-hydrogen elimination reactions. On the contrary, the sequential loss of methyl radicals seems the dominant path of (CH3)(2)Se-2, beta-like hydrogen elimination reactions being still possible, although less likely than for (C2H5)(2)S-2. The occurrence of a peculiar CH3. transposition reaction is also suggested for (CH3)(2)Se-2. Weak or negligible [for (C2H5)(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 (CH3)(2)Se-2 [(C2H5)(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) x 10(17) cm(-3), are found by secondary ion MS in ZnSe samples and attributed to the low proclivity of (CH3)(2)Se-2 to decompose through beta-like hydrogen reactions.