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Understanding paleoenvironmental conditions, including the frequency and amplitude of seasonal variation, is crucial to understanding a wide array of human behaviors in prehistory. In vertebrate calcified tissue, inorganic oxygen isotopes derived from apatite have been widely used as a proxy for paleoenvironment and seasonality in the archaeological record. However, organic sources of delta O-18 and delta D in mineralized tissue have remained largely unexplored. Here we measure the oxygen (delta O-18) and hydrogen (delta D) composition of dentin collagen sequentially sampled from the lower cheek teeth of four modern ovicaprids from the Baga Gazar’in Chuluu (BGC) region of the Gobi Desert of Mongolia. A significant correlation between trends in hydrogen and oxygen isotope values within individual teeth was identified. The amplitude of isotopic variation observed in sampled tooth dentin is similar to the yearly range of hydrogen and oxygen isotopic variation observed in meteoric precipitation for the region. These data indicate that dentin collagen delta O-18 and delta D isotopic values reflect seasonal variation in the organic oxygen and hydrogen isotopic composition of diet and ingested water. We argue that paired measurements of dentin collagen delta O-18 and delta D appear to be reliable seasonal climatic indicators. (C) 2008 Elsevier Ltd. All rights reserved.