Stable linear weakly bound hydrogen-bonded complexes of HArF and HKrF with the CO molecule have been predicted by ab initio computations at the MP2/6-311++G(2d,2p) level of theory. The complexes, having stabilities in the order, FArH...CO>FKrH...CO>FArH...OC>FKrH...OC are compared. They exhibit unusual blueshifts of the Ar-H (Kr-H) stretching frequency, as well as contraction of the Ar-H (Kr-H) bond, and these effects are rationalized mainly in terms of shifts in the electron density of HArF (HKrF) on complexation, caused mainly by a combination of the intermolecular electrostatic interaction, electron-electron (Pauli) and nuclear-nuclear repulsion and charge density transferred from the CO molecule to the rare-gas-containing molecule. (C) 2004 American Institute of Physics.