  We examine the long-time behaviour of A+B \to 0 reaction-diffusion systems
with initially separated species A and B. All of our analysis is carried out
for arbitrary (positive) values of the diffusion constant D_A of particles A
and initial concentrations a_0 and b_0 of A's and B's. We derive general
formulae for the location of the reaction zone centre, the total reaction rate,
and the concentration profile of species A outside the reaction zone. The
general properties of the reaction zone are studied with a help of the scaling
ansatz. Using the mean-field approximation we find the functional forms of
`tails' of the reaction rate R and the dependence of the width of the reaction
zone on the external parameters of the system. We also study the change in the
kinetics of the system with D_B > 0 in the limit D_B \to 0. Our results are
supported by numerical solutions of the mean-field reaction-diffusion equation.
