Theoretical and experimental investigations of the Danish produced DM subsurface moisture probe have been carried out at the Research Establishment Risö, and the results obtained are presented in this paper. The DM probe contains an Am-Be fast neutron source and has a glass scintillator containing 6Li as neutron detector. Through modification of the probe it was possible to vary the source-detector distance in steps. By the use of a three-group diffusion model and a semi-empirical calibration procedure good agreement between measured and calculated calibration curves was obtained. Further the model has been used to study the effect of variations in the dry soil density and in the boron content of soil. Finally the applicability of the model in connection with probe design has been considered.