Modulated-beam mass spectrometry and temperature programmed desorption have for the first time been used to investigate the decomposition of tri-isopropyl gallium (TIPGa) on the (100) GaAs surface. Significant differences in the substrate temperature dependence of the GaAs growth rate have been observed between growth using TIPGa and the standard CBE precursor triethylgallium (TEGa). These differences are explained in terms of the balance between desorption and decomposition of adsorbed di-isopropyl gallium radicals shifting in favour of desorption.