In this study, the influence of tetrafluoromethane and fluoroform on the phase behaviour of methane gas hydrates has been investigated. The pressure and temperature conditions for the phase boundary hydrate-liquid water-vapour(H-L-w-V) are determined experimentally for vapour phase compositions varying from 0 to 1 in the ternary systems water + methane + tetrafluoromethane, water + methane + fluoroform, and water + carbon dioxide + fluoroform. Additionally the phase boundary hydrate-liquid water-liquid fluoroform (H-L-w-L-g) has been determined for the ternary systems containing fluoroform in a limited range of vapour phase compositions. For the experiments Cailletet equipment has been used, that allows studying the phase behaviour in a pressure range from 1 to 14 MPa, and the temperature range considered was 273-298 K approximately. The experimental data were modelled using the van der Waals and Platteeuw model, which included a description of the fluid phases with the Soave-Redlich-Kwong equation of state and correction for the solubility of gaseous components in the liquid phase with Henry's law according to the Krichevsky-Kasarnovsky relation. The system water + methane + tetrafluoromethane shows azeotropic behaviour and forms structure I gas hydrate in the investigated vapour phase composition range. The pressures for the phase boundary H-Lw-V of this system can be predicted within 6%. The modelling work of the water + methane + fluoroform system shows that at the H-L.-V phase boundary, structure I is formed, while there are indications that at the H-L-w-L-g phase boundary a change from structures I to II occurs. Equilibrium pressures of this system can be predicted within 15%. (c) 2006 Elsevier B.V. All rights reserved.