[en] Laboratory studies have been conducted of ion-molecule reactions involved in the synthesis of NH₃, H₂O, and HCN in dense interstellar clouds, and the rate constants and product distributions for these reactions have been determined. The rate constant for the reaction NH₃⁺+H₂→NH₄⁺+H is extremely slow at interstellar cloud temperatures, and this has some important consequences for mechanisms of NH₃ synthesis in these regions. Atomic C⁺ ions formed in dense clouds by the reaction He⁺+CO subsequently react with ammonia to produce the H₂CN⁺ ion, leading to the synthesis of HCN. The HCN is subsequently destroyed in turn by reaction with C⁺. In the case where HCN is both synthesized and destroyed by reaction of C⁺ ions, a simple, model-independent expression can be obtained for the relative abundances of HCN and NH₃. When we use our laboratory results for the rate constants and product distributions of the appropriate reactions, the ratio [HCN]/[NH₃]approx. =0.4 is predicted. This result in consistent with the observations. Thermoneutral exchange between H₂CN⁺ ions and HD is shown to be very slow, but the high [DCN]/[HCN] ratio observed in interstellar clouds (Wilson et al.) can be explained by enhancement in the [H₂D⁺]/[H₃⁺] ion ratio. On this basis, a large deuterium enhancement is also predicted for the ''X-ogen'' lines, [DCO⁺]/[HCO⁺]approx. =10^-3 to 10^-2