  Mixtures of relativistic gases are analyzed within the framework of Boltzmann
equation by using Grad's moment method. A relativistic mixture of $r$
constituent is characterized by the moments of the distribution function:
particle four-flows, energy-momentum tensors and third-order moment tensors. By
using Eckart's decomposition and introducing $13r+1$ scalar fields -- related
with the four-velocity, temperature of the mixture, particle number densities,
diffusion fluxes, non-equilibrium pressures, heat fluxes and pressure deviator
tensors -- Grad's distribution functions are obtained. Grad's distribution
functions are used to determine the third-order tensors and their production
terms for mixtures whose constituent's rest masses are not too disparate, so
that it follows a system of $13r+1$ scalar field equations. By restricting to a
binary mixture characterized by the six fields of partial particle number
densities, four-velocity and temperature, the remainder 21 scalar equations are
used to determine the constitutive equations for the non-equilibrium pressures,
diffusion fluxes, pressure deviator tensors and heat fluxes. Hence the
Navier-Stokes and generalized Fourier and Fick laws are obtained and the
transport coefficients of bulk and shear viscosities, thermal conductivity,
diffusion, thermal-diffusion and diffusion-thermal are determined. Furthermore,
solutions of the relativistic field equations for the binary mixture are
obtained in form of forced and free waves. In the low frequency limiting case
the phase velocity and the attenuation coefficient are determined for forced
waves. In the small wavenumber limiting case it is shown that there exist four
longitudinal eigenmodes, two of them corresponding to propagating sound modes
and two associated with non-propagating diffusive modes.
