The TOCATTA model simulates 14C transfer within terrestrial and/or continental water ecosystems, in response to chronic and accidental hypothetical emissions of 14C, in a liquid and/or gaseous form. This model has been developed from bibliographical knowledge on C cycling and common hypothesis to existing 14C transfer models, in order to come within the conceptual and mathematical frameworks of the SYMBIOSE project. In this context, the conceptual modelling deals with discretizing the continental biosphere into elementary components likely to be contaminated and identifying interactions or transfer processes of 14C between two components. The mathematical modelling for calculating stocks and fluxes under accidental conditions is based on a system of differential equations expressing conservation of pollutant activity for each component of the conceptual model. It is shown that the analytical solutions obtained when temporal derivatives of the differential equation system are equal to zero correspond to the activities at “steady-state" simulated for chronic contaminations.