Ab initio calculations were performed at the SCF, B3LYP, MP2, QCISD, and CCSD(T) levels of theory using the cc-pVTZ basis set to obtain optimized geometries, dipole moments, binding energies, and harmonic vibrational frequencies for the linear complexes HCN . . . HF and HNC . . . HF. It was found that HNC . . . HF has a slightly larger binding energy [29 kJ mol(-1) at CCSD(T)] than HCN . . . HF [27 kJ mol(-1) at CCSD(T)]. The relative stabilities of the monodeuterated isotopomers DCN . . . HF (DNC . . . HF) and HCN . . . DF (HNC . . . DF) were also determined from their zero-point energies; vibrational frequency shifts of HF and HCN (HNC) on bonding were also calculated and used to gain insight into these relative stabilities. It was found that the D-bonded isotopomer is more thermodynamically stable than the H-bonded isotopomer. Previous studies on the weakly bound linear complexes CO-acetylene and N-2-acetylene also found that the D-bonded species was more stable than the H-bonded species.