An exact expression for diffusion time, which depends on the interaction rates for particles of not only different, but also of the same species, has been derived from the system of kinetic equations. The result is valid for particles with arbitrary statistics and energy–momentum relations. The derived general relations are valid for investigating diffusion in liquid and solid3He–4Hemixtures. The contribution of interaction between quasiparticles of the same type to the diffusion coefficient and effective thermal conductivity of superfluid solutions is analyzed. The calculated values are compared with experimental data. The calculated diffusion coefficient of3He–4Hesolid solutions differs from the previous theoretical results. A comparison of the obtained diffusion coefficient with experimental data makes it possible to determine the numerical value of the energy band width for impurity quasiparticles. ©1997 American Institute of Physics.