Monte Carlo modelling studies of three different passive neutron-counting chambers: JCC51, JCC62 and FUNE are presented in this paper. The qualification process of the models consists of performing qualitative and quantitative comparisons between simulation and measurement for a 252Cf calibrated source. Parametric sensitivity studies allow an evaluation of some of the uncertainties due to modelling hypotheses. The simulation of the response of the coincidence counting systems is done using the codes MCNP and SOURCES4C, and the “two-parameters method”. The consistency of the calculation scheme is here evaluated with five plutonium oxide samples, perfectly characterized concerning their mass and isotopic composition. The first results obtained with those characterized or calibrated materials show that MCNP can provide a total count with a deviation of about 3% compared to the measured value, and the combined MCNP / two-parameters method can provide total counts with about 5% and real counts with about 15% of deviation compared to the measured value.