In this research, new experimental data of hydrate equilibrium conditions were measured for the following systems: tributylphosphine oxide (TBPO) + water; TBPO + CO2 + water; TBPO + CH4 + water; TBPO + CO2 + CH4 + water. The gas mixture CO2 + CH4 had a composition of 40 mol% of CO2 in order to simulate a typical biogas produced by an anaerobic digestion of agricultural waste. Differential scanning calorimetry (DSC) method was applied to measure the dissociation temperatures for each system. The dissociation enthalpy per mole of gas was estimated using Clausius-Clapeyron approximation. For the binary system, the mass fraction of TBPO in aqueous solution varied from 0.05 to 0.40 and the phase transition temperature varied from 278.1 K to 281.0 K. For the ternary systems the measurements were carried out for a mass fraction of TBPO varying from 0.05 to 0.30 in the gas pressure range of 1-3 MPa. The measured dissociation temperatures varied from 284.4 K to 292.5 K. The results showed that the presence of TBPO reduces the gas pressure for hydrate phase formation compared to the binary systems (gas + water). The higher dissociation temperature of TBPO/CH4 semi-clathrate, compared to TBPO/CO2 semi-clathrate, was confirmed. The phase behavior diagram of the binary and the ternary systems were also determined, which would be of interest to determine the operating conditions of potential industrial processes involving hydrate formation, such as gas storage or gas separation. (C) 2016 Elsevier Ltd.