Al2O3 atomic layer deposition (ALD), which uses trimethylaluminum (TMA) as the metal precursor, shows promise in improving the environmental stability of hybrid halide perovskites. However, it is not yet entirely clear how TMA, a strong Lewis acid, reacts with fresh perovskites and how the reaction affects the nucleation of ALD Al2O3. Here, the effects of reaction temperature and partial pressure of TMA on the mechanisms of TMA/CH3NH3PbI3 reactions are investigated. Our real time mass gain data and in situ mass spectrometry data show that the TMA/CH3NH3PbI3 reaction can either remove mass or accumulate mass onto CH3NH3PbI3 substrates, depending strongly on the reaction temperature and partial pressure of TMA. The TMA/CH3NH3PbI3 reaction probably generates TMA-CH(3)NHx adduct compounds, which protects CH3NH3PbI3 from TMA by forming a shell at 25 degrees C in the vacuum process. However, these adduct compounds decompose at higher temperatures (e.g., 75 degrees C). This product layer is much thicker than a monolayer, suggesting the interface formed between Al2O3 coating and CH3NH3PbI3 is blurring and messy. These results have not yet, but should be, carefully considered to correctly interpret the effect of ALD Al2O3 treatment on optoelectronic properties of CH3NH3PbI3.