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.