Interest has grown in tetra-n-butylammonium bromide (TBAB) semi-clathrates due to their formation at lower pressures and higher temperatures than conventional gas hydrates. This study focuses on TBAB semi-clathrates formed with carbon dioxide and methane and is to our knowledge the first study to report and model semi-clathrate former solubility under hydrate-liquid-vapor equilibrium conditions. A thermodynamic model was developed using the Trebble-Bishnoi equation of state to calculate vapor and liquid phase fugacity. Liquid activity was determined using the electrolyte non-random two liquid model (eNRTL). The solid semi-clathrate phase was modeled using a modified van der Waals-Platteeuw model (vdW-P), with parameters re-optimized based on the data obtained in this study. Equilibrium conditions were successfully modeled over the temperature range of 281-294 K, pressure range of 377-11,000 kPa and TBAB composition range of 5-40 wt%. The model functioned well for carbon dioxide with an average absolute relative error (AARE) of 4.7% for pressure prediction and an AARE of 17.5% for solubility. Methane was more difficult to model and yielded AAREs of 21.6% for pressure and 32.5% for solubility. Further understanding of semi-clathrate crystal structure and interactions between methane and TBAB would aid in improving the model. (C) 2014 Elsevier B.V. All rights reserved.