[en] A theoretical model is developed for the epithermal neutron porosity tool. It is based on three-group diffusion-theory. All neutron groups are in the fast neutron region, and the lowest energy group is that in which the neutrons are the detected by the detector. The model is one-dimensional, and it assumes an Am-Be point source. In order to investigate the importance of the various design parameters the model has been used for parameter studies. The parameters considered were: Type of matrix material, source-detector distance, active detector length and energy range of the detected neutrons. Calibration curves were calculated both for water-saturated formations and for formations with constant matrix density. In the latter case various density values were considered. The studies revealed that the most important design parameter is the source-detector distance. The model was compared with available, experimental data. While it was possible to obtain reasonable agreement between measurements and calculations for a source-detector distance of 28.5 cm, this values does not agree with the source-detector distance of 40 cm given in literature. Thus further investigations are needed. (author)