ZnSe epitaxial films have been grown on (100) GaAs by reduced pressure organometallic chemical vapor deposition (OMCVD) from tertiary-butyl(allyl)selenium (tBASe) and dimethylzinc triethylamine adduct (DMZnNEt3) at temperatures of 325-450ΰC. Good surface morphology, film crystallinity and interface quality have been found with scanning electron microscopy (SEM), double crystal X-ray diffraction (DCD) and Rutherford back scattering spectroscopy (RBS). Secondary ion mass spectrometry (SIMS) shows negligible carbon concentration (below 5x1017atoms/cm3). Low temperature photoluminescence (PL) exhibits a strong near band-edge emission with a dominant donor-bound peak. Gas-phase pyrolysis of tBASe has been probed at reduced pressure in a molecular beam mass spectrometric system in hydrogen and deuterium carrier gases. The precursor decomposes above 200ΰC by β-hydrogen elimination and by homolysis of the Se-C bonds. High isobutene vs. isobutane ratios (50-100) indicate a predominance of β-hydrogen elimination over homolysis at temperatures below 400ΰC. Diallylselenium is present in the gasphase in low concentrations at temperatures of 200-350ΰC. Diallylselenium, methylallylselenium and dimethyl-selenium have been observed as minor by-products during pyrolysis of co-dosed tBASe and DMZnNEt3. The effect of the retro-ene decomposition pathway of allylselenium reagents on carbon incorporation into ZnSe films is further probed by growth experiments with in situ generated 2-methylpropaneselenal.