Abstract: The carbon and oxygen isotope compositions of enamel carbonate hydroxylapatite from mammalian fossils are used to reconstruct paleoenvironment. δ^13C values are indicators for diets and δ^18O values are indicators for water sources of mammals. Commonly, carbonate hydroxylapatites are reacted with pure phosphoric acid at 25 - 90℃ for 10 minutes to 3 days to liberate CO2 gas for analysis. In this study, 23 experiments were performed at diverse reaction temperatures, reaction times or pretreatments. Enamel, dentin and chalk from fossils and enamel from modern equids were collected and 23 δ^13C and δ^18O values were compared. HAC-H2O2-HAC or HC1-H2O2-HAC is more effective than HCl-H2O2-NaHSO4 or HCl-H2O2-Na2B4O7to purify carbonate hydroxylapatites, and high temperature such as 70℃ is more advisable than low temperature, especially for δ^13℃ values