  We propose a simple model for reaction-diffusion systems with orientational
constraints on the reactivity of particles, and map it onto a field theory with
upper critical dimension d_c=2. To two-loop level the long-time particle
density N(t) is given by the same universal expression as for a nonsteric
system, with N(t)~t^{-d/2} for d<=2. For slow rotations of the particles we
find an intermediate regime where N(t)~t^{-d/4}, with the crossover to the
nonsteric asymptotics determined by the rates of rotations and reactions.
Consequences for experiments are discussed.
