In order to get a better understanding of the phase diagrams of natural condensate gases, three synthetic mixtures of known composition were investigated. The major components were methane (approximately 79 mol%), and pentane (approximately 20 mol%). A heavy hydrocarbon fraction (0.5 mol%) was composed either of n-tetracosane, naphthalene or both components. The detailed phase diagram of each mixture was determined in a fully visible sapphire cell, from 10 to 40 MPa and from 277 to 420 K. This study confirms the shape of the (gas)/(gas + solid) and (gas + liquid)/(gas + liquid + solid) phase boundaries. These phase boundaries converge at a single common point with the (gas)/(gas + liquid) and (gas + solid)/(gas + liquid + solid) phase boundaries. The investigated fluids also illustrate the various possible locations of the (gas = liquid) critical point. The phase diagrams could be modelled qualitatively with the Peng-Robinson equation of state, assuming that the heavy hydrocarbons crystallised as pure phases. The influence of pressure and of solid/solid phase transitions is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.