Here we report an investigation of ultrathin tin oxide films on Pt3Sn(111) using low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (μ-LEED), scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and high-resolution X-ray photoelectron spectroscopy (XPS). Oxidation at ∼390–410 °C produces triangular, two-dimensional oxide islands that nucleate rapidly and exhibit self-limited lateral growth, attributed to limited Sn diffusion from the subsurface of the crystal. μ-LEED shows that the initially formed (4×4) Sn oxide is subsequently converted to a more oxygen-rich (2×2n) “stripe” phase. At 630 °C, enhanced Sn mobility enables a closed (4×4) film. The (2×2n) phase is shown to consist of a (2×2) Sn lattice modulated by 1D stripe defects with spacings of n=4–6 atomic rows; LEED and SXRD measurements show diffraction features corresponding to this striped superstructure. The two oxides can be distinguished in XPS by their O 1s lineshapes: the (4×4) phase shows a clear doublet attributable to distinct O species, whereas the (2×2n) phase exhibits a broader envelope consistent with a distribution of O coordination environments. The Sn 3d5/2 spectra are similar for both phases, reflecting closely related Sn bonding motifs. The spectra are consistent with those of previous near-ambient-pressure XPS measurements, suggesting that the surface oxides forming under CO oxidation conditions are similar to those studied here.