The carbon isotope (delta ¹³C) signature of CO₂ within fluid inclusions is of significance to investigate because it can be used to decipher the carbon source of geological fluids. Raman spectroscopy has been applied to determine the delta ¹³C of pure CO₂. However, whether this approach can be applied to natural inclusions with complex compo-sitions remains unknown. Here we present Raman spectra of CO₂±N₂±CH₄ systems with known ¹³C/¹²C ratio (0.01-1.12) at temperatures (T) of 21 degrees C and 40 degrees C and at pressures (P) ranging from 0.5 to 50 MPa. Peak areas and peak heights of the ¹³CO₂ (~1370 cm^-1) and ¹²CO₂ (~1285 cm^-1) bands were determined to calculate the Raman spectroscopic quantification factors (F-and G-factor ratio). The results show that the quantification factors vary significantly with rising pressure in the low pressure range (e.g., <10 MPa), especially for pure CO₂ system. Different fluid systems are characterized by pressure dependences. Specifically, both F-and G-factor ratios for CO₂ fluid with a low ¹³C/¹²C ratio (e.g., 0.01) are different from that with a high ¹³C/¹²C ratio (e.g., 0.13) under the same T-P conditions. Fluid temperature does not result in significant variation in F-or G-factor ratios at the investigated temperatures. Based on these calibrations, we suggest that fluid pressure and compo-sition (x) are prerequisites for Raman spectroscopic measurements of the ¹³C/¹²C ratio of CO₂. However, the ¹³CO₂ (~1370 cm^-1) band for CO₂ from natural fluid inclusions is characterized by low Raman intensity, resulting in large errors in calculated peak area and peak height. Thus, accurate determination of the ¹³C/¹²C ratio of fluid inclusion CO₂ is not feasible for inclusions with low CO₂ density. Nevertheless, this method can be applied to investigate the carbon isotope fractionation in ¹³C labelled hydrothermal experiments, where CO₂ with high a ¹³C/¹²C ratio is produced.