Semi-clathrate hydrates of tetra-n-butyl ammonium bromide (TBAB) offer a potential solution for separation of CO2 from biogas. In this work, gas hydrate formation from a CO2/CH4 mixture in distilled water with 0.05 gig (5 wt%) TBAB was examined at 275, 277, and 279 K with pressures of 4000,'5000, and 6000 kRa. The compositional changes of the gas phase were analyzed continuously during the hydrate formation process. The results indicated that no significant induction occurred during hydrate formation. The CO2 separation factor increased with decreasing temperature and increasing pressure. The maximum amount of CO2 gas molar ratio in the residual gas phase was 79.81 mol % at 4000 kPa and 275 K with a gas/liquid volume ratio (V-g/V-I) of 1. TBAB promoted hydrate formation and CO2 separation; however, the CO2 separation factor decreased in a decomposed solution compared with fresh TBAB solution. The experimental results provide a theoretical basis for the use of hydrate basedgas separation (HBGS) technology in biogas purification.