We discuss the physical significance of the parameters Δklthat arise from mostly the adiabatic and nonadiabatic effects in the vibration-rotational energles of diatomic molecules for which data from isotopic variants are available. Examples are given of the use c f these parameters to derive radial functions for HC1 and LiH. Similar to those for the Uklcoefficients, relations among the Δklcoefficients have been developed, but caution is urged in their use.