AlN films were grown by both plasma and thermal ALD from TMA and NH3, while the effects of process temperature on precursor reaction mechanisms were studied in situ by mass spectrometry. The surface chemical reactions were monitored at m/z = 16, corresponding to CH4 + , which was produced in both the TMA and NH3 pulses. The results from mass spectrometry and the obtained GPC support recent DFT modelling that the reaction pathway for TMA chemisorption on AlN is reductive elimination. From mass spectrometry results, we suggest that the NH3 chemisorption pathway involved ligand exchange to produce CH4. Our results also indicated that the reaction activities were much slower for the TMA half-cycle, making it the rate-limiting step.