A new method for joint sti!ness determination has been developed, with applications in tool/toolholder interfaces. Based upon rigid-body dynamics and frequency response function (FRF) measurements, the method allows simultaneous determination of sti!ness components in six coordinate directions (three translations, three rotations), resulting in a 6;6 sti!ness matrix for the joint. The method circumvents several limitations of many common test procedures, including the requirement for large force application and the restriction of single-direction measurements. Inertia properties of the joined test pieces, easily calculated due to their simple geometry, are used along with FRF data to tune the model, and to determine the forces and moments acting through the joint. This information is combined with relative displacement measurements (derived from measured acceleration data) to determine the sti!ness matrix. The product of simple and well-established impact testing techniques, the results validate the procedure as a straight-forward approach for isolating a joint's dynamics and measuring its elastic properties. The procedure is demonstrated on a sample tool/toolholder interface, and the sti!ness results are reported.