Tin halide perovskites (THP) have gained much attention in the field of thin film photovoltaics over the past few years, owing to their bandgap of 1.4 eV, which is close to the ideal bandgap for single-junction devices according to the Shockley-Quessier limit.[1] Despite their great promise and high photostability,[2] devices based on THP still suffer from poor efficiencies mainly due to the so-called self-p doping effect, the easy Sn²⁺ oxidation and high instabilities when exposed to oxygen in air. We have previously shown that the presence of high background hole doping in pristine THPs (i.e., without additives) usually dominates their charge-carrier recombination dynamics,[3] introducing pseudo-monomolecular radiative recombination pathways of photoexcited electrons with background holes, resulting in very high external photoluminescence quantum yields and ultrashort carrier lifetimes.[4] One of the most used approaches to limit the self p-doping of THPs is the addition of SnF₂, which effectively reduces the background hole doping density and simultaneously passivates Sn surface trap states (Sn⁴⁺) thanks to the beneficial role of fluorine.[3] By combining measured and simulated charge-carrier dynamical information with x-ray photoelectron spectroscopy and computational modelling, we provide a comprehensive picture of the role of SnF₂ additive in THP thin films. The addition of SnF₂ reduces the peak associated to Sn⁴⁺ species and promotes the formation of a new peak that we assign to the presence of F interstitial on the surface, hindering the degradation of the absorber and preventing the formation of SnI₄, which instead acts as electron trap.  We further investigate the evolution of both optoelectronic properties and surface chemistry of THP films with and without SnF₂ as additive when exposed to oxygen in air. Air exposure induces a simultaneous increase in the doping level and in the intensity of deep electron trap states, which can be assigned to an increased formation of energetically shallow tin vacancy defects and SnI₄ degradation products on the surface, respectively. Interestingly, the presence of SnF₂ within the film does not alter nor slow down the degradation mechanism. The interstitial F species disappear upon air exposure in favour of SnI₄, SnO₂ and segregated SnF₂ species. To conclude, we demonstrate that the addition of SnF₂ during film preparation can serve as an oxygen scavenger and hinder the degradation of the absorber, while it does not affect the degradation mechanism when the film is exposed to air.