Abstract The diffusion parameters in diphenyl (C 10H 10) at 77°C and MIPB (C 15H 16) at 30°C were determined using the pulsed neutron source technique. The 1 MeV Van de Graaff accelerator at North Carolina State College, Raleigh, was used as the pulsed source of fast neutrons utilizing the 9Be (d, n) 10B reaction. The decay of the thermal neutron leakage from a finite cylindrical geometry was measured using a 26-channel time analyser. By measuring the thermal neutron decay constant, A, as a function of buckling, B 2, a least squares fit of the data to the equation λ = λ 0 + D 0 B 2 − CB 4 yielded values of D 0, the diffusion constant, and C, the diffusion cooling coefficient. The infinite medium decay constant, λ 0, was calculated from known cross-section data. For diphenyl measurements in the buckling range from 0·0816 cm −2 to 0·2811 cm −2, the diffusion parameter values obtained were D 0 = 5437 ± 0117 × 10 4 cm 2-sec −1 and C = 9·85 ± 5·50 × 10 3 cm 4-sec −1. The results obtained for MIPB in the buckling range from 0·0945 cm −2 to 0·2952 cm −2 were D 0 = 3780 ± 0·033 × 10 4 cm 2-sec −1 and C = 125 ± 1·43 × 10 3 cm 4-sec −1. The values of the transport mean free path, λ tr, and diffusion length, L, derived from these results were found to be less than those values predicted by current theoretical models. A preliminary investigation of λ tr, in MIPB as a function of temperature showed a non-linear behaviour of the transport mean free path with temperature.