This paper reviews some of the non‐equilibrium processes governing the dynamics of molecular formation and destruction in the interstellar medium. In the first part, it is shown how the existence of interstellar grains is mainly responsible for the formation of H2and how a small fractional ionization of H and H2by cosmic rays explains satisfactorily how most of the simple common molecular species are subsequently formed by a series of exoergic ion‐molecule reactions. The second part of the review is devoted to the problem of the fragmentation of molecules by the interstellar photons whose energy is less than the ionization potential of atomic H. For most of the common interstellar molecules, photodissociation occurs via an indirect mechanism, due to non‐adiabatic coupling between a bound valence state with a dissociative channel. The resonant character of the process leads to a very frequency selective photodissociation cross section, which can induce isotopic fractionation. A brief description of the theoretical methods used to treat photodissociation induced by non adiabatic coupling is discussed. The results of our calculated photodissociation cross sections for the CH+molecule are compared with recent experiments using an ion storage ring. The excellent agreement between theory and experiment confirms that such non‐adiabatic processes are well understood. © 2004 American Institute of Physics