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A theoretical and experimental analysis of the phase change of a volatile liquid in an unsaturated soil is presented. Expression of the rate of phase change of the pore liquid is developed within the framework of nonequilibrium thermodynamics. An experiment is proposed in which the value of the phenomenological coefficient associated with this expression is measured. Two examples of results concerning the volatilisation of water and of a volatile hydrocarbon in a silty clayey sand illustrate the potentiality of the experiment. Variation of the phase change coefficient was determined at 30°C for both fluids and at 80°C for heptane, versus the liquid content of the soil.