Tracer transport in two-phase (liquid-gas) conditions is a complex process that involves advection, diffusion, and hydrodynamic dispersion. Volatile tracers may be strongly affected by partitioning between gas and liquid phases, and by exchanges between fractures and rock matrix. This paper presents a space-discretized treatment of tracer diffusion under multiphase conditions that is fully coupled with equilibrium phase partitioning. Our method is self-consistent and is applicable to any combination of gas and liquid phases in single and two-phase conditions, including the extreme case of diffusion across a sharp gas-water interface. It is shown that an uncoupled approach, in which diffusive fluxes in gas and liquid phases are evaluated separately and added, can lead to large errors. The coupled treatment of multiphase tracer diffusion has been implemented in the general-purpose geothermal reservoir simulator TOUGH2. Applications to strongly and weakly depleted zones at The Geysers vapor-dominated reservoir produce breakthrough curves (BTCs) whose features (timing, peak, tail) agree well with field observations. Published by Elsevier Science Ltd on behalf of CNR.