Knowledge of the CO2 concentration in the unsaturated zone is essential for prediction of solution chemistry in the vadose zone and groundwater recharge as well as for quantifying carbon source/sink terms as part of the global CO2 mass balance. In this paper we present a predictive simulation model, SOILCO2, based on process-oriented relationships. The model includes one-dimensional water flow and multiphase transport of CO2 utilizing the Richards and the convection-dispersion equations, respectively, as well as heat flow and a CO2 production model. The transport of CO2 in the unsaturated zone can occur in both the liquid and gas phases. The gas transport equation accounts for production of CO2 and uptake of CO2 by plant roots associated with root water uptake. The CO2 production model considers both microbial and root respiration which is dependent on water content, temperature, growth, salinity and plant and soil characteristics. Heat flow is included, since some gas transport parameters, partitioning coefficients and production parameters are strongly temperature dependent. The resulting set of partial differential equations is solved numerically using the finite element and finite difference methods.