We develop a model of the diffuse interstellar cloud along the line of sight to o Per. The model comprises a region of gas in equilibrium, under the action of the background radiation field, a C-type shock and post-shock gas. With the exception of CH, the observed column densities of a range of atomic and molecular species, including CH + and H 2 (J= 4), can be successfully reproduced. The elemental abundances of C, N and O need to be depleted by a factor of approximately 0.4. The influence of the charge transfer reaction O (H + , O + ) H on the ortho:para-H 2 ratio is discussed