A number of mono-, bi- and tricyclic 4-piperidones were prepared and their NMR and IR spectra measured. The spectroscopic parameters show that the strain associated with these heterocycles decrease in the order tricyclic > bicyclic > monocyclic. It appears that in the case of the bicyclic system, part of the strain is relieved by one of its six-membered rings assuming a boat form and by flattening whereas in the tricyclic system the strain cannot be relieved to a significant extent. Copyright © 2001 John Wiley & Sons, Ltd.