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The common oxides of plutonium are the dioxide (PuO 2 ) and the sesquioxide (Pu 2 O 3 ). The nature of an oxide on plutonium metal under air at room temperature is typically described as a thick PuO 2 film at the gas-oxide interface with a thinner Pu 2 O 3 film near the oxide-metal substrate interface. In a reducing environment, such as ultra high vacuum, the dioxide (Pu 4+ ; O/Pu=2.0) readily converts to the sesquioxide (Pu 3+ ; O/Pu=1.5) with time. In this work, the growth and evolution of thin plutonium oxide films is studied with x-ray photoelectron spectroscopy (XPS) under varying conditions. The results indicate that, like the dioxide, the sesquioxide is not stable on a clean metal substrate under reducing conditions, resulting in substoichiometric films (Pu 2 O 3-y ). The Pu 2 O 3-y films prepared exhibit a variety of stoichiometries (y~0.2-1) as a function of preparation conditions, highlighting the fact that caution must be exercised when studying plutonium oxide surfaces under these conditions and interpreting resulting data.