A model is presented describing the factors and processes which determine the measured 14C ages of soil calcium carbonate. Pedogenic carbonate forms in isotopic equilium with soil CO2. Carbon dioxide in soils is a mixture of CO2 derived from two biological sources: respiration by living plant roots and respiration of microorganisms decomposing soil humus. The relative proportion of these two CO2 sources can greatly affect the initial 14C content of pedogenic carbonate: the greater the contribution of humus-derived CO2, the greater the initial 14C age of the carbonate mineral. For any given mixture of CO2 sources, the steady-state 14CO2 distribution vs. soil depth can be described by a production/diffusion model. As a soil ages, the 14C age of soil humus increases, as does the steady-state 14C age of soil CO2 and the initial 14C age of any pedogenic carbonate which forms. The mean 14C age of a complete pedogenic carbonate coating or nodule will underestimate the true age of the soil carbonate. This discrepancy increases the older a soil becomes. Partial removal of outer (and younger) carbonate coatings greatly improves the relationship between measured 14C age and true age. Although the production/diffusion model qualitatively explains the 14C age of pedogenic carbonate vs. soil depth in many soils, other factors, such as climate change, may contribute to the observed trends, particularily in soils older than the Holocene. © 1994.