Mixed halide perovskites have attracted a strong interest in the photovoltaic community as a result of their high power conversion efficiency and the solid opportunity to realize low-cost and industry-scalable technology. Light soaking represents one of the most promising approaches to reduce non-radiative recombination processes and thus to optimize device performances. Here, we investigate the effects of 1 sun illumination on state-of-the-art triple cation halide perovskite thin films Cs_0.05(MA_0.14, FA_0.86)_0.95 Pb (I_0.84, Br_0.16)₃ by a combined optical and chemical characterization. Competitive passivation and degradation effects on perovskite transport properties have been analyzed by spectrally and time-resolved quantitative imaging luminescence analysis and by X-ray photoemission spectroscopy (XPS). We notice a clear improvement of the optoelectronic properties of the material, with a increase of the quasi fermi level splitting and a corresponding decrease of methylammonium MA⁺ for short (up to 1 h) light soaking time. However, after 5 h of light soaking, phase segregation and in-depth oxygen penetration lead to a decrease of the charge mobility.