Gas separation and gas storage involving semi-clathrate formation are promising processes that can be used in many industrial fields. Among them, there is the possibility to purify the biogas, which is a gas mixture composed essentially of carbon dioxide (CO2) and methane (CH4). The investigations of the feasibility of these processes require knowledge about the phase diagram of the involved system. This work presents the phase diagrams of the systems of tetra-n-butyl phosphonium bromide (TBPB) + H2O, CH4 + TBPB + H2O and CO2 + CH4 + TBPB + H2O. The dissociation temperatures of the semi-clathrate phase were determined at a mass fraction of TBPB varying from 0.05 to 0.35 and at a 1-8 MPa pressure range. For CH4 + TBPB + H2O systems the measured temperatures varied from 280.8 to 293.1 K. The dissociation temperature of the semi-clathrate formed with TBPB has also been obtained in the case of CH4 + CO2 mixtures, the proportion of the gases being close to what is observed in typical biogases. (C) 2015 Elsevier B.V. All rights reserved.