Organometallic vapor phase epitaxy (OMVPE) has achieved remarkable recent success, becoming the most promising technique for the ultimate production of III/V materials and device structures. Unfortunately, our understanding of the growth process remains primitive. In this paper we report a new technique for tracing the reactions by conducting the epitaxial growth in a D2 ambient using a time-of-flight mass spectrometer to analyze the product molecules. The pyrolysis reactions were studied for PH3, both alone and in the presence of trimethylindium (TMIn), and for TMIn alone and in the presence of PH3. For the reactants alone, the PH3 pyrolysis is completely heterogeneous at the InP surface, while TMIn pyrolyzes homogeneously in the gas phase. For TMIn and PH3 together, the reaction mechanism is entirely different; the pyrolysis temperatures for both PH3 and TMIn are lowered. Since the reaction produces only CH4 molecules, with a complete absence of CH3D at high ratios of PH3 to TMIn, we hypothesize that InP growth is initiated by the direct interaction of TMIn and PH3 molecules, either in the vapor phase at normal growth temperatures or at the InP surface at temperatures as low as 250°C.