Abstract The measurements and calculations have been made for D 2O moderated 252Cf spherical assembly of diameter 30 cm which has been adopted as a calibration standard for the testing of personal neutron dosimeters. The leakage neutron spectrum has been measured in the energy range from 0.1 to 10 MeV using a proton recoil spectrometer and compared with MCNP calculations with ENDF/B-IV and ENDF/B-VI nuclear data files. The experimental and calculational results were compared also with the earlier reference data recommended by ISO. The Cf sources were placed to the center of D 2O sphere in a channel of diameter 8 or 25 mm. The dependence of leakage neutron spectrum on channel diameter was measured and calculated. The influence of H 2O admixture has been calculated. The results indicate generally better agreement of measurements and calculations using ENDF/B-VI data than using ENDF/B-IV. Nevertheless, some differences between measurements and calculations with ENDF/B-VI in certain energy regions were still observed if the measurements were repeated with different spectrometers.