  The observed H$_2$ line ratios in OMC--1 and IC443 are not satisfactorily
explained by conventional shock excitation models. We consider the microscopic
collisional processes implicit in ambipolar diffusion models of magnetised
C-shocks and show that non-LTE level populations and emission line ratios are
an inevitable consequence of such models. This has important implications for
the use of molecular hydrogen lines as diagnostics of shock models in molecular
clouds.
