The mechanism of phonon scattering by precipitates in metal alloys was investigated by lattice thermal conductivity measurements, at low temperatures, of age hardened Al-Zn-Mg, Al-Cu-Mg and Cu-Co alloys. Electrical resistivity measurements in the same temperature range enabled the electronic contribution to the thermal conductivity to be deduced by employing the Wiedemann-Franz law; this procedure was shown to be acceptable, within experimental accuracy, even for the Cu-Co system which exhibited an anomalous minimum in the electrical resistivity results. An electron microscopic investigation of the aluminium alloys was carried out to examine the dislocation and precipitate distributions in the aged specimens, and to correlate them with the thermal conductivity results. The formation and growth of the precipitates in the Cu-Co specimens were followed by magnetic measurements, utilizing the superparamagnetic behaviour showm by this system in the aged condition. A theoretical model, based on a transport scattering cross-section derived for a spherical elastic inclusion embedded in an isotropic elastic continuum, provided reasonable agreement with the experimental results obtained for the copper-cobalt alloy.