  The kinetics of the q species pair annihilation reaction (A_i + A_j -> 0 for
1 <= i < j <= q) in d dimensions is studied by means of analytical
considerations and Monte Carlo simulations. In the long-time regime the total
particle density decays as rho(t) ~ t^{- alpha}. For d = 1 the system
segregates into single species domains, yielding a different value of alpha for
each q; for a simplified version of the model in one dimension we derive
alpha(q) = (q-1) / (2q). Within mean-field theory, applicable in d >= 2,
segregation occurs only for q < 1 + (4/d). The only physical realisation of
this scenario is the two-species process (q = 2) in d = 2 and d = 3, governed
by an extra local conservation law. For d >= 2 and q >= 1 + (4/d) the system
remains disordered and its density is shown to decay universally with the
mean-field power law (alpha = 1) that also characterises the single-species
annihilation process A + A -> 0.
