The effects of biodegradation and heating on the stable carbon, nitrogen, hydrogen, and oxygen isotope ratios of chitin in arthropods were studied. Chitinous exoskeletons from seven aquatic arthropod species were subjected to anaerobic marine biodegradation in mud, to terrestrial biodegradation in soils, and to thermal degradation under anaerobic and aerobic conditions. The isotope ratios of chromatographically separated D-glucosamine hydrochloride and derivatives from treated and untreated specimens were then compared. Isotope ratios were found to be conserved during partial degradation of chitin. Micromorphological comparative studies using scanning electron microscopy and transmission electron microscopy indicated that no fungal chitin or other contaminants were present in the chitins that were chemically isolated from biodegraded substrates. Results indicate that it will be possible to use stable isotope ratios of archaeological chitin samples for environmental and climatic reconstructions. Isotope ratios of chitin from crustacean exoskeletons recovered from archaeological sites with ages up to 1400 years BP are in good agreement with measurements on modern crustaceans from similar environments. See also <i>AATA<b> 23-1151</i> for earlier, related study. -- AATA