Abstract An interstellar reaction scheme of the type described by E. Iglesias (Astrophys. J. 218, 697 (1977)) has been extended to include deuterium chemistry and also isomeric forms of some molecules. The role of isomeric forms of the intermediates CNH2 and HCNH+ in the production of interstellar HCN and HNC is considered. The lowest triplet states of these ions probably play an important part in determining the proportions of HCN and HNC produced. The results of numerical integration of the 108 coupled kinetic equations involved in the extended scheme are presented as time-dependent plots of concentrations of the different chemical species. Calculated concentrations at a molecular cloud age of 10 Ma (1 Ma — 106 years) are within the experimental uncertainties for 11 of the 14 species for which suitable observational data are available. Predicted values of HX /DX ratios fall into three broad groups around 102, 103 and the accepted cosmic H /D ratio, 105, and observations are broadly in agreement. Some reported enrichments that are higher than the predicted figures may arise from the use of data for optically thick molecular lines. Some previously unpublished observations of DNC and HN13C illustrate doubts associated with optically thick lines.