Small crystals of NaI(TI) were exposed to (D, D) neutrons slowed down in a paraffin moderator. The crystals were either enclosed in cadmium or placed inside a small cadmium box filled with the moderator material. The activities induced in iodine at different positions were measured. Now let the normalized activities obtained with the detectors enclosed in cadmium be A1 and let those obtained with the detectors placed inside the cadmium box be A2. It is assumed that the difference A2 − A1 is proportional to q(0.5 eV)-the slowing down density at an energy of 0.5 eV-and from the measured quantities A2(r) and A1(r) the slowing down age has been determined. Bare crystals were also irradiated and the induced activities measured. Let these activities be A3. Then the difference A3 − A1 will give a measure of the density of thermal neutrons and the migration area can be determined. With migration area and slowing down age known, the diffusion length of thermal neutrons could be determined too. The result obtained was L = (2.86 ± 0.16)cm compared to a calculated value of (2.60 ± 0.01)cm. The discrepancy might indicate that the neutron temperature in the paraffin is about 400 °K. Paraffin was used as moderator material because this greatly facilitated the experiment. It is never used as a reactor material and knowledge of the measured quantities may be of minor importance. The method, however, might possibly be of some interest.