The carbonation process in lime mortar is influenced by the diffusion of carbon dioxide into the mortar pore system, by the kinetics of the lime carbonation reaction, and by the drying and wetting process in the mortar. All these phenomena depend on the presence of water in the mortar. Water is present in liquid form as well as vapour. Physical models and their mathematical basis are given for the factors influencing carbonation. The mathematical description of the carbonation process in lime mortar completes the information necessary to model the simulation of carbonation in conjunction with the drying process. Simulations at different carbon dioxide concentrations showed a good correlation between experiment and mathematical prediction.