Present theories for the formation of molecular species observed in comets predict the sublimation of parent molecules such as H2O, CH4, CO2, and NH3, from the surface of the nucleus and their subsequent photodissociation and ionization to form the observed species (Delsemme 1973). It can be shown (Oppenheimer 19 75) that gas phase chemical reactions occur between these fragments which have characteristic timescales which are short compared to the timescale for significant variation in the solar flux incident on the comet. Hence, a steady-state approximation may be used for determining the densities of many species. It can also be shown (Oppenheimer 1975) that the rate of formation of many species is faster by gas phase reactions than by photoprocess. For instance, the formation of OH+ from H2O+ by the reaction

H2O+ + O + → OH+ + OH