RATIONALE: The conventional method for measuring carbon and oxygen isotopes in carbonates involves the reaction of carbonate minerals with phosphoric acid (PPA) to generate CO 2, followed by purification and isotopic analysis. During this reaction, a temperature-dependent oxygen isotope acid fractionation factor (AFF) is introduced, as not all oxygen is released in CO 2. While the temperature dependence of AFF has been extensively studied in pure carbonate minerals, little research has been conducted on AFF variations in mixed carbonate minerals with diverse chemical and mineralogical compositions. This study aims to investigate the range of AFF variations in carbonates containing multiple mineral phases. METHODS: Oxygen isotope compositions of CO 2 produced from CaCO 3-MgCO 3 mixed minerals reacting with PPA at 25℃, 50℃, 70℃, 90℃ were analyzed using a GasBench II system coupled with a Thermo Finnigan Delta V Plus isotope ratio mass spectrometer. RESULTS: The AFF values for CaCO 3-MgCO 3 mixed minerals fall between those of pure calcite and dolomite, generally aligning with theoretical predictions. Additionally, the fractionation gradient (dδ 18O/dT -1) increases with rising Mg/Ca ratio and dolomite content, indicating a systematic trend in fractionation behavior. CONCLUSIONS: These findings provide a framework for AFF correction based on Mg/Ca ratio or dolomite content, enhancing the reliability and precision of oxygen isotope measurements in natural impure carbonates.