The effect of local phonon modes, due to light impurities in a heavy host, on the electrical resistivity, is calculated. The theory of Kagan and Zhernov (1966) is extended to include inelastic impurity scattering as well as the energy dependence of the electronic distribution function. An analysis of the various effects causing the deviations from Matthiessen's rule (DMR) shows that, although a distinctive signature of the local modes exists, it will usually be overwhelmed by other effects causing DMR.