Abstract
A short review is given of measured interstellar abundances in both hot (T > 20 K) and cold (T < 20 K) molecular clouds. The data are interpreted under the assumption that in both cases, the chemistry may be relatively far from chemical equilibrium. The observed abundances probably reflect initial conditions produced by some event such as a shock or the ‘switch-on’ of a star which radically changes both the physical parameters of the interstellar material and the fraction of heavy elements in the gas phase. The difference between hot-cloud and cold-cloud chemistry is partly due to the nature of such initial events and partly due to the subsequent evolution of gas-phase chemistry.
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