  We present a model for the hydrogenation states of Polycyclic Aromatic
Hydrocarbons (PAHs) in the diffuse interstellar medium. First, we study the
abundance of hydrogenation and charge states of PAHs due to photo-ionization,
photo-dissociation in the interstellar UV field, electron recombination and
chemical reactions between PAH cations and H or H_2. For PAH cations, we find
that the dehydrogenation effects are dominant. The hydrogenation state of PAHs
depends strongly on the H density, the size of the molecule and UV field. In
diffuse clouds with low H density and normal UV radiation, PAHs containing less
than 40 C are completely or strongly dehydrogenated whereas at high H density,
they are normally hydrogenated. The partially dehydrogenated species dominate
in intermediate density clouds. PAHs above 40 C are quite stable and are fully
hydrogenated, which would favor their spectroscopic search in near IR surveys
of Diffuse Interstellar Bands (DIBs).
