The effects of depletion on the elements C, N and O on the chemical composition of dark clouds are investigated, using both isothermal and isochoric cloud models. This work differs from previous approaches in that a much larger range of CNO depletions is considered. The chemistry of the ortho- and para-forms of H 2 and the exothermic reaction between N + and ortho-H 2 , which synthesizes NH 3 , were included. In the isothermal models, the ortho:para ratio is very small at large depletions, but NH 3 formation is still efficient owing to reactions between He + and CN or HCN. In the isochoric models, the equilibrium temperature of the gas is larger, and a thermal ortho:para ratio, which is large enough to drive NH 3 formation, results