The phase behavior of tetrabutylphosphonium bromide (TBPB) semi-clathrates with three gases, viz. CO2, CH4 or N-2 was investigated in this study. The measurements were performed using a static high pressure cell using the isochoric pressure search technique. In order to determine the thermodynamic stability conditions of the semi-clathrates, the equilibrium data were measured at various concentrations of TBPB, viz. (0.05, 0.075, 0.10, 0.15, and 0.20) mass fraction TBPB. Measurements were undertaken in the temperature range of (279.3-291.4) K and in the pressure range of (1.06-7.79) MPa. The results indicate that TBPB has a promotion effect on semi-clathrate hydrate formation for the pure gas systems under consideration. (C) 2015 Elsevier B.V. All rights reserved.