A technique was developed for determining the previously unknown thermal-neutron diffusion parameters, ∑a and D, of a small amount of a moderating material, perfluorodimethylcyclohexane, C8F16. A two-region cylindrical system consisting of C8F16 in the core region and water in the reflector region was pulsed with neutrons to obtain the decay constants for several heights of the system. By comparing these results with those obtained from a similar water-graphite experiment and by utilizing one-group, two-region diffusion theory, it was possible to obtain the desired parameters for the fluoro-carbon, i.e., D(C8F16) = 1.02 cm, ∑a(C8F16) = 90 × 10−5/cm, and L(C8F16) = 33.6 cm. This technique offers a convenient method for determining the neutron diffusion parameters of small samples of rare materials.