The HCNH+ + HCN <----> HNC + HCNH+ proton exchange reaction has been investigated by means of quantum chemical calculations with the RHF, MP2, CCSD(T), and B3LYP methods. The overall process corresponds to a proton-assisted HCN <----> HNC isomerization whose activation barrier is about 10 kcal/mol lower than that of the corresponding 1,2-hydrogen shift. Furthermore, we found that the HNC + HCNH+ reverse reaction represents a possible low-energy channel for the depletion of HNC in molecular clouds. This mechanism might be involved within the regulation of the HCN/HNC abundance ratio in the interstellar space. (C) 2002 Published by Elsevier Science B.V.