Bubble-point curves, dew-point curves and solid + fluid/fluid boundary curves of methane + tetracosane + triacontane were determined according to the synthetic method in the temperature range 320-475 K and at pressures up to 200 MPa. Two series of quasi-binary systems of constant methane mole fraction with mixtures of tetracosane and triacontane with composition varying between pure tetracosane and pure triacontane were studied. One quasi-binary system with methane mole fraction 0.975 showed dew-point behaviour, the other quasi-binary system with methane mole fraction 0.900 showed bubble-point behaviour. A gradual isothermal substitution of tetracosane by triacontane at constant mole fraction methane increases the bubble-point pressure and the dew-point pressure, while the liquidus temperature at constant pressure passes through a minimum. Similar behaviour was found by Floter et al. [E. Floter, B. Hollanders, Th.W. de Loos, J. de Swaan Arons, Fluid Phase Equilib. 143 (1998) 185-203] in the system methane + docosane + tetracosane. (c) 2004 Elsevier B.V. All rights reserved.