  A diffusion-limited annihilation process, A+B->0, with species initially
separated in space is investigated. A heuristic argument suggests the form of
the reaction rate in dimensions less or equal to the upper critical dimension
$d_c=2$. Using this reaction rate we find that the width of the reaction front
grows as $t^{1/4}$ in one dimension and as $t^{1/6}(\ln t)^{1/3}$ in two
dimensions.
