Tetrabutyl ammonium bromide (TBAB) forms a semi-clathrate hydrate, which can incorporate small gas molecules such as methane and nitrogen. It has recently been used for separation of gases. However, there are very limited experimental data on the phase boundaries of the gas hydrate form in the presence of TBAB. In this work, we present new experimental data at high-pressure TBAB, w = 0.10, TBAB (w = 0.10 and 0.43) + hydrogen, TBAB (w = 0.05, 0.10, 0.20, and 0.30) + methane, TBAB (w = 0.10) + nitrogen, TBAB (w = 0.1 and 0.427) + carbon dioxide, and TBAB (w = 0.05, 0.10, and 0.43) + natural gas semi-clathrate hydrate phase boundaries. In another part of this work, the results of visual observations of the methane + TBAB semi-clathrate hydrate morphology and the methane gas bubbles released from methane + TBAB semi-clathrate hydrates on dissociation are presented. Finally, the effect of TBAB mass fraction on hydrate promotion and the stability of the new semiclathrate hydrate are presented. © 2007 American Chemical Society.