The effect of the deposition temperature and the partial pressure of water on the thermal decomposition chemistry of aluminium-tri-sec-butoxide (ATSB) during metal organic chemical vapour deposition (MOCVD) is reported. The MOCVD experiments were performed in nitrogen at atmospheric pressure. The partial pressure of ATSB was 0.026 kPa (0.20 mmHg) and that of water was between 0 and 0.026 kPa (0–0.20 mmHg). The pyrolytic decomposition chemistry of ATSB was studied by mass spectrometry at temperatures between 160 and 420°C. The effect of water on the homogeneous reaction products was studied at 270, 300, 330 and 370°C. The main products were 2-butanol, n-butane, 2-butanone, 1-butene and/or 2-butene, and water. The amount of by-products increased with increasing temperature. Water did not significantly affect the homogeneous product formation, whereas the heterogeneous deposition reaction was extensively reduced with increasing partial pressure of water, above 270°C. From the type, amount and distribution of the products formed during the pyrolytic decomposition of ATSB, an attempt was made to establish the main decomposition mechanism: free radical, -hydride elimination, or β-hydride elimination.