Packaging methylammonium lead iodide perovskite (MAPbI(3))-based solar cells with N-2 Or dry air is a promising solution for its application in outdoor photovoltaics. However, the effect of N-2 and O-2 on the decomposition chemistry and kinetics of MAPbI(3) is not yet well-understood. With in situ Fourier transform infrared spectroscopy measurements, we show that the effective activation energy for the degradation of MAPbI(3) in N-2 is similar to 120 kJ/mol. The decomposition of MAPbI(3) is greatly accelerated by exposure to O-2 in the dark. As a result of the synergistic effect between O-2 and a HeNe laser (633 nm), the degradation rate is further increased with photon flux. This synergistic effect reduces the effective activation energy of degradation of MAPbI(3) to similar to 50 kJ/mol. The solid decomposition products after annealing in N-2 and O-2 at 150 degrees C or below donot have absorbance between 650 and 4000 cm(-1).