The transition between Ta2O5 and TaO2 governs resistive switching in tantalum oxide-based resistive random access memory. Despite its importance, the Ta2O5–TaO2 transition is scarcely described in the literature, in part because the tantalum oxide layer in devices is amorphous, which makes it difficult to characterize. In this paper, we use first-principles calculations to construct the convex hull of the amorphous Ta2O5−x system for 0 ≤ x ≤ 1 and show that oxygen deficiency in tantalum oxide leads to phase-separation into Ta2O5 and TaO2. In addition, our work challenges the conventional interpretation of X-ray Photoelectron Spectroscopy (XPS) spectra of the Ta 4f orbitals. Specifically, we find that TaO2 exhibits both the Ta4+ peak associated with TaO2 and the Ta5+ peak normally associated with Ta2O5. While our simulated Ta2O5 peak originates from a narrow range of oxidation states, the TaO2 peak comes from disproportionated Ta atoms with Bader charges ranging from +3 to +1, the lowest of which are well below Ta atoms in crystalline TaO. Finally, we demonstrate that the XPS blueshift of around 1 eV observed experimentally in amorphous Ta2O5 with respect to crystalline Ta2O5 comes from both the presence of under-coordinated Ta atoms and longer Ta–O bond distances in the amorphous system. Our simulated XPS analysis shows that amorphous XPS spectra may be more complex than previously thought, and hence, caution should be applied when assigning XPS peaks to oxidation states.