Zr-2.5wt.% Nb alloy specimens were exposed to various high temperature environments and the oxides formed were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using specimen preparation techniques specifically designed to examine the oxide-metal interface. The high temperature environments included oxygen at 300-degrees-C, air at 490-degrees-C, steam at 490-degrees-C and pressurized water (containing 4.8 g LiOH/l H2O) at 300-degrees-C. The oxidation rate in 300-degrees-C oxygen gas was lower than 490-degrees-C steam or 300-degrees-C pressurized water. The oxidation rate was highest for the 490-degrees-C air environment. Examination of the oxide at or near the oxide-metal interface showed that specimens with the lowest oxidation rate (300-degrees-C oxygen) had a compact oxide film with a fine grain size (almost-equal-to 10 nm), whereas the specimens with the higher oxidation rates had an oxide film with interconnected porosity.