In paper 1 of this two-paper series (S˘imůnek and Suarez, this issue) we presented a description of the numerical model, SOILCO2, for CO2 transport and production in the unsaturated zone. In paper 2 the model sensitivity to various parameters is evaluated by both steady state and transient simulations, with a range in the parameter values typically found under field conditions. We also select parameter values for optimal plant and microbial CO2 production and production dependence on temperature, water content, osmotic potential and gas composition for plant and microbial respiration, all based on literature review. The predictive capabilities of the SOILCO2 model are evaluated by comparing model simulations to published field data from Missouri for three different crops and two growing seasons under transient conditions as well as a data set collected in Riverside, California, under relatively constant water content at depth. The model provided good prediction of the CO2 flux to the atmosphere as well as the concentrations in the root zone for all data sets.