  A collisionless shock may be strongly modified by the presence of neutral
atoms through the processes of charge exchange between ions and neutrals and
ionization of the latter. These two processes lead to exchange of energy and
momentum between charged and neutral particles both upstream and downstream of
the shock. In particular, neutrals that suffer a charge exchange downstream
with shock-heated ions generate high velocity neutrals that have a finite
probability of returning upstream. These neutrals might then deposit heat in
the upstream plasma through ionization and charge exchange, thereby reducing
the fluid Mach number. A consequence of this phenomenon, that we refer to as
the "neutral return flux", is a reduction of the shock compression factor and
the formation of a shock precursor upstream. The scale length of the precursor
is determined by the ionization and charge exchange interaction lengths of fast
neutrals moving towards upstream infinity. In the case of a shock propagating
in the interstellar medium, the effects of ion-neutral interactions are
especially important for shock velocities < 3000 km/s. Such propagation
velocities are common among shocks associated with supernova remnants, the
primary candidate sources for the acceleration of Galactic cosmic rays. We then
investigate the effects of the return flux of neutrals on the spectrum of
test-particles accelerated at the shock. We find that, for shocks slower than
~3000 km/s, the particle energy spectrum steepens appreciably with respect to
the naive expectation for a strong shock, namely E^-2.
