In this study, we evaluate the variation of the work function of phosphorene during thermal oxidation at different temperatures. The ultraviolet photoelectron spectroscopy results show an N-shaped behaviour that is explained by the oxidation process and the dangling-to-interstitial conversion at elevated temperatures. The exfoliation degree and x-ray photoelectron spectroscopy confirm the formation of native oxides in the top-most layer that passivates the material. Ex-situ XPS reveals the full oxidation of monolayers at temperatures higher than 140 C, but few-layer phosphorene withstands the thermal oxidation even up to 200 C with slight modifications of the A 2 g/A 1 g and A 2 g/B 2g vibrational mode ratios and a weak fluorescence in the Raman spectra of the heat-treated samples.