A mathematical model of multiphase transport of heat, mass, and reacting chemical species in a porous media is developed. The model is appropriate for describing many of the reactions that take place in a geothermal reservoir. This model leads to the separation of transport terms from reaction terms in the equations for chemical species concentration. A numerical method for the solution of the transport equations is developed based on the integrated finite difference method. Four example problems representative of those of interest in geothermal reservoirs are solved. These examples consider up to 14 chemical species and nine chemical reactions among these species. ¿ American Geophysical Union 1995