A recently introduced valence-state potential energy function (von Szentpaly, L. Chem. Phys. Lett. 1995, 245, 209.) and its dissociation energy (D-vs) serve to establish an arithmetic combining rule for scaled force constant increments, 1 = kR(e)/D-vs. lambda is transferable from homo- to heteronuclear molecules at an accuracy of 1.6% average absolute error in harmonic vibrational wavenumbers, (v) over tilde(e), of 42 diatomic molecules covering a force constant range of factor 30. The parallel to combining rules for repulsive potentials in solid-state and atomic physics is shown, and the reasons for the failure of previous attempts to obtain transferable force constant increments are examined.