The production of microbially derived CO2 was investigated in the 3.6 m thick unsaturated geologic media of a mesoscale model system. The mesoscale model, which was 2.4 m diameter¿4.6 m high, was filled with a medium grained lacustrine sand, capped with an Ap (0.11 m) and B (0.20 m) soil profile, and maintained under steady state moisture conditions for 343 days. A near steady state soil gas CO2 concentration profile (>100 times greater than atmospheric) developed after 130 days implying that geochemical stability was rapidly attained. The presence of metabolically active microorganisms, consumption of O2 and dissolved organic carbon, and carbon 13 data indicated that the CO2 was microbially produced. At steady state, the calculated and modeled CO2 flux to the atmosphere (about 0.9 mol m-2 day-1) were greater than the measured CO2 flux (0.1 mol m-2 day-1) and greater than the calculated CO2 flux to the water table (0.02 mol m-2 day-1). The difference between the calculated and measured fluxes of CO2 to the atmosphere was attributed to spatial variability in the gas diffusion coefficient and reflected the difficulty estimating CO2 production, even under steady state conditions. Laboratory estimates of microbial CO2 fluxes (1.8 and 2.9¿103 mol m-2 day-1) were much greater than those determined from measured and calculated fluxes. Measured microbial respiration rates suggested that between 50 and 70% of the respired CO2 was produced in the unsaturated geologic medium beneath the Ap and B, and that there was sufficient CO2 production potential by bacteria to account for the respired CO2 determined from the flux calculations. These observations indicated that long-term experimentation in mesoscale models can overcome some of the difficulties encountered in field studies and provide insight into biogeochemical processes in natural systems. ¿ American Geophysical Union 1993