This work was done in the framework of the International Intercomparison "Uncertainty Assessment in Computational Dosimetry: A Comparison of Approaches" organised by the European Radiation Dosimetry Group (EURADOS) .The aim of the intercomparison was to study the theoretical response of a proposed widespread neutron spectrometer exposed to arbitrary neutron sources. The modelled neutron spectrometer is a Bonner Sphere Spectrometer (BSS), that consists of 8 spheres made of high-density polyethylene (δ=0.95gcm3 ), with diameters between 2” and 12 “ in addition to a 12”diameter sphere loaded with lead, with an active thermal neutron detector (6 LiI(Eu)) located at each sphere centre. The theoretical neutron response was calculated using MCNPX code, version 2.6d using ENDF/B-VI and LLNL libraries in the energy range between thermal neutron and 17.77 MeV for monoenergetic beams and ISO standard 241Am-Be and 252Cf sources.