Rationale: Aerodyne Tunable Infrared Laser Differential Absorption Spectrometer (TILDAS) allows for the measurement of carbonate-clumped (∆ 638) and stable oxygen isotope ratios (δ 628) without the mass interference of 17O, and understanding acid fractionation factors during phosphoric acid digestion is essential for inter-laboratory data comparison. Here, we present δ 628 and ∆ 638 ratios in CO 2 generated during phosphoric acid digestion of a reference carbonate at different temperatures. Methods: Carrara Marble (MAR-J1) calcite is digested with ⁓104% phosphoric acid using a) Break Seal method is a modified version of a McCrea-type reaction vessel where complete equilibration is achieved between product CO 2 and H 2O generated from the digestion and b) Individual Acid Bath (IAB) where the samples are digested with fresh aliquot of acid each time and the product CO 2 and H 2O simultaneously frozen in a U-trap connected to the reaction chamber by liquid N 2. Results: The regression equations for acid fractionation of δ 628 are: a) Break Seal Method: 1000 ln α (dig. temp- 25℃) = (0.532 ± 0.030) × 10 6/T 2 + (-6.185 ± 0.308) ; R 2 = 0.98. b) IAB: 1000 ln α (dig. temp- 25℃) = (0.359 ± 0.027) × 10 6/T 2 + (-4.114 ± 0.247) ; R 2 = 0.97. The regression equations for acid fractionation of ∆ 638 are: a) Break Seal Method: 1000 ln α (dig. temp - 25℃) = (0.0189 ± 0.0017) × 10 6/T 2 + (-0.2151 ± 0.0174) ; R 2 = 0.95. b) IAB: 1000 ln α (dig. temp - 25℃) = (0.0529 ± 0.0041) × 10 6/T 2 + (-0.6072 ± 0.0356) ; R 2=0.97. Conclusion: The shallow slope of the IAB method for δ 628 and the steeper slope for ∆ 638 indicate minimal resetting of the isotope composition of product CO 2 due to re-equilibration or interaction with free H 2O molecules. This observation enables inter-laboratory comparisons of δ 628 and ∆ 638 in calcite.