In this work, we report the first ab initio study of the photochemistry of some chromium(III) ammine complexes: Cr(NH3)5F2+, trans-Cr(NH3)4X2+ (X = F-, Cl-), and cis-Cr(NH3)4F2+. A dissociative mechanism is adopted and the photoinduced substitution reactions are considered to be the result of three separate processes: (1) the selective loss of a ligand from the complex in its first excited state, (2) the isomerization of the so-formed pentacoordinated fragment, and (3) the association of this fragment with an entering solvent ligand. For all complexes, the ab initio method leads to a correct identification of the photoactive state. Moreover, the electron density shifts upon excitation can be correlated rather nicely with the preferential leaving ligand as observed experimentally. For Cr(NH3)5F2+ and trans-Cr(NH3)4Cl2+ the isomerization pathways of the relevant pentacoordinated fragments are studied by using Woodward-Hoffmann state correlation diagrams. These diagrams are further combined into a single reaction surface of a Jahn-Teller type. The stereomobility of the Cr(III) photochemical substitution reactions is rationalized in terms of a single rule which describes the deactivation pathways on this surface. The ab initio results are in remarkable agreement with the earlier ligand field calculations.