The Rapid Chloride Migration test (NT Build 492) is commonly used in order to determine the chloride diffusion (migration) coefficient in concrete. This coefficient can then be used as the input value for service life design models for concrete structures in marine environment. However, the theoretical background of this test is still raising discussions as the equations describing the transport process of chlorides cannot predict correctly the chloride concentration profiles in concrete. The new, improved chloride transport model presented in this paper includes the nonlinear nature of chloride binding in concrete as well as the non-equilibrium conditions between free- and bound- chloride concentrations, which were not taken into account in the traditional chloride transport model. This new model has been applied to the experimental data and the values of binding parameters, chloride mass transfer coefficient and the effective chloride diffusion coefficient were determined in the optimization process. The derived binding parameters (non-linear Freundlich isotherm) show very good agreement with values obtained from experimental measurements. The chloride mass transfer coefficient reveals a tendency to diminish during the RCM test. The effective diffusion coefficient determined based on the new chloride transport model is in line with the values found in literature.