The desire to improve the corrosion resistance of Zr cladding material to allow high burnup has resulted in a general trend among fuel manufacturers to develop alloys with reduced levels of Sn. Whereas the detrimental effect of Sn on high-temperature aqueous corrosion performance is widely accepted, the reason for it remains unclear. High-energy synchrotron x-ray diffraction was used to characterise the oxides formed by autoclave exposure on Zr-Sn-Nb alloys with tin concentrations ranging from 0.01 to 0.92 wt. %. The alloys studied included the commercial alloy ZIRLO and two variants of ZIRLO with significantly lower tin levels, referred to here as A-0.6Sn and A-0.0Sn. The nature of the oxide grown on tube samples from each alloy during autoclave testing at 360°C was investigated by cross-sectional scanning and transmission electron microscopy (SEM and TEM). Non-destructive synchrotron x-ray diffraction analysis on the oxides revealed that the monoclinic and tetragonal oxide phases display highly compressive in-plane residual stresses with the magnitudes dependent on both phase and alloy. Additional in situ synchrotron x-ray diffraction experiments during oxidation at 550°C provided further confirmation of the trends seen for autoclave-tested samples and demonstrated the presence of elevated levels of tetragonal phase in the initial stages of oxidation. In situ and ex situ measurements demonstrate unambiguously that the amount of tetragonal phase present and, more importantly, the degree of transformation from tetragonal to monoclinic oxide both decrease with decreasing tin levels, suggesting that tin stabilises the tetragonal phase. It is proposed that in Zr-Nb-Sn alloys with low Sn, the tetragonal phase is mainly stabilised by very small grain size and, therefore, remains stable throughout the corrosion process. By contrast, in alloys with higher tin levels, larger, stress stabilised, tetragonal grains can form initially, but then transform as the corrosion front progresses inward and stresses in the existing oxide relax.