Tin halide perovskites represent the only realistic route toward lead-free perovskite optoelectronics. Despite significant progress, however, the device efficiency and stability of solar cells are still limited by the perovskite self-p-doping and by Sn(II) oxidation to Sn(IV). By employing state-of-the-art density functional theory simulations, we unveil the mechanistic features and energetics of Sn(II) → Sn(IV) oxidation in pristine and defective models. Surprisingly, tin oxidation is predicted to be considerably unfavorable in bulk MASnI3 while it is energetically favored at unpassivated perovskite surfaces. As a consequence, bulk Sn(IV) spontaneously transforms into Sn(II), releasing two holes to the valence band and p-doping the perovskite, while surface Sn(IV) acts as a deep electron trap and contributes to nonradiative carrier recombination. The stoichiometry and the valence band surface pinning are found to largely …