SUMMARYSturm-Liouville equations will be considered where the boundary conditions depend rationally on the eigenvalue parameter. Such problems apply to a variety of engineering situations, for example to the stability of rotating axles. Classes of these problems will be isolated with a rather rich spectral structure, for example oscillation, comparison and completeness properties analogous to those of the 'usual' Sturm-Liouville problem which has constant boundary conditions. In fact it will be shown how these classes can be converted into each other, and into the 'usual' Sturm-Liouville problem, by means of transformations preserving all but ÿnitely many eigenvalues.