A simple relation is presented between the bond lengthening, δr, and the vibrational frequency decrease for the Morse oscillator H-X coupled by the van der Waals bond to a structureless moiety B. Estimates of δr(HX) in the series of complexes B · HX (X = F,Cl) range from 0.0002 Å for Ar · HCl to 0.04 Å for Me2O · HF. For the B · HF dimers, the predicted values of δr(HF) are in excellent agreement with the theoretical results reported by Liu and Dykstra. © 1989.