  A quasiparticle description of various condensed media is a very popular tool
in study of their transport and thermodynamic properties. I present here a
microscopic theory for the description of diffusion processes in two-component
gas of quasiparticles with arbitrary dispersion law and statistics.
Particularly, I analyze the role of interaction within each subsystem (i.e.
between identical quasiparticles) in relaxation of the whole system. The
approach for solution of such kinetic problem allows to study the most
important limiting cases and to clarify their physical sense. Classical results
for diffusion coefficient of light particles in a massive gas (Lorentz model)
and of massive particles in a light gas (Rayleigh model) are obtained directly
from the general solution without using artificial approaches, as it was done
earlier. This particularly provide a possibility to generalize these popular
models on quasiparticle systems.
