The chloride and CO2 chemistry of the Wairakei geothermal field, New Zealand, has been modelled using an extended version of the geothermal simulator TOUGH2. This version of the simulator Solves the equations for the transport of reacting chemical species in multi-phase fluids, and is applied here to a detailed, full-scale geothermal reservoir model for the first time. Reactions involving the speciation of CO2 to H2CO3 and HCO3- are included in the model, as is the ＇＇Henry＇s Law＇＇ reaction for exsolution of aqueous CO2 to the vapour phase. Because CO2 speciation in water is pH dependent, a reaction involving the most important weak acid buffer at Wairakei (H4SiO4) has also been included in the model. The chloride is treated as a conservative, nonreacting species that is present only in the liquid phase. Results from the model compare favourably with measured chloride and CO2 data from Wairakei covering the period 1959-1987.