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Nitrates minerals from the Dry Valleys of Antarctica have been analyzed for their oxygen and nitrogen isotopic compositions. The N-15 was depleted with delta N-15 values ranging from - 9.5 to - 26.2 parts per thousand, whereas the O-17 and O-18 isotopes were highly enriched (with excess O-17) with delta O-18 values spanning 62 - 76% and Delta O-17 values from 28.9 to 32.7%. These are the largest O-17 enrichments observed in any known mineral. The oxygen isotopes indicate that nitrate is from a combination of tropospheric transport of photochemically produced HNO3 and HNO3 formed in the stratosphere.