  An overview is given of the chemical processes that occur in primordial
systems under the influence of radiation, metal abundances and dust surface
reactions. It is found that radiative feedback effects differ for UV and X-ray
photons at any metallicity, with molecules surviving quite well under
irradiation by X-rays. Starburst and AGN will therefore enjoy quite different
cooling abilities for their dense molecular gas. The presence of a cool
molecular phase is strongly dependent on metallicity. Strong irradiation by
cosmic rays (>100x the Milky Way value) forces a large fraction of the CO gas
into neutral carbon. Dust is important for H2 and HD formation, already at
metallicities of 10^-4-10^-3 solar, for electron abundances below 10^-3.
