This N model is derived from the C model (MOMOS) published by Sallih and Pansu (1993). Both models were fitted to experimental data obtained from an incubation experiment of 1 4 C- and 1 5 N-labelled plant material in two soils with contrasting characteristics over 2 y under controlled laboratory conditions. The N model uses the same structure as the C model with five organic compartments: labile plant material (V L ); stable plant material (V R ); microbial biomass (B); labile humified material (A); and stable humified material (H). Two additional compartments are included: exchangeable NH 4 -N and NO 3 -N. The transfers of N between the organic compartments are described according to first-order kinetics. Nitrogen transferred to the NH 4 compartment results from the balance between the output and input of all the organic compartments. Ammonium N output is split between nitrification and immobilisation into B (77%) and H (23%). Nitrification is controlled by a microbial growth law. The N model uses the parameters defined in the C model, with a constant multiplying factor for the N kinetic constants (fn = 1.4 and 1.9 in soil 1 and 2, respectively). The additional parameters defined for inorganic pools are comparable in both soils. The predicted 1 4 C-to- 1 5 N ratios of each organic compartment agree with the experimental data, showing a rapid incorporation of 1 5 N into microbial biomass and a gradual build-up into stable humified compounds. The model was adjusted to 1 5 N experimental data from five time series (each series containing from five to eight sampling occasions), and was validated using five series of corresponding total N data. MOMOS-C and -N formulation is relatively simple, combining mechanistic first-order kinetic models and growth models. The predictions are in agreement with 16 data sets including different forms of organic 1 4 C (three series), total C (three series), organic and inorganic 1 5 N and total N.