For a mixture of chemical species labelled by indices  the equation for species  is
where , and the collision term is
where , the magnitude of the relative momenta is
and  is the differential cross-section, as before, between particles i and j. The integration is over the momentum components in the integrand (which are labelled i and j). The sum of integrals describes the entry and exit of particles of species i in or out of the phase-space element.