In this study, the equilibrium data of the hydrates forming in tetra-n-butylphosphonium bromide (TBPB) aqueous solution with and without carbon dioxide (CO2) are measured at mass fractions w = (0.10, 0.20, 0.35, and 0.50) in the pressure range of (0.4 to 4.0) MPa and in the temperature range of (281.0 to 292.0) K. Meanwhile, the crystal morphologies of TBPB hydrates with and without CO2 are visually observed. The equilibrium data and morphologies of TBPB hydrates are compared with those of tetra-n-butylammonium bromide (TBAB) hydrates.