We have constructed a computer model for the precipitation kinetics of niobium carbonitrides in ferrite. The model describes the time evolution of each age-class of precipitates. This procedure allows for the full coupling of the physical laws for nucleation, growth and coarsening phenomena. The originality of this work is to take into account the composition evolution of the precipitates with time, thus allowing for the prediction of both effects of carbon and nitrogen in solid solution. Specific techniques such as chemical analysis after selective dissolution and transmission electron microscopy with electron energy loss measurement (TEM-EELS) have been used for a thorough investigation of the precipitation state. Time, temperature and composition dependence have been explored. The measured precipitate composition, size and number of particles compare satisfactorily with the calculated values.