The effect of longitudinal harmonic oscillations on free convective heat transfer from an electrically heated vertical plate is considered. Experiments have been performed for frequencies ranging from 10 to 20 cps at an amplitude of 0·25 in. at plate temperatures varying from 100 to 200°F. The temperature profiles in the boundary layer have been measured by a Mach-Zehnder interferometer. The results have been presented in terms of the local values of Nusselt number, Nu x , Grashof number, Gr x and vibrational Reynolds number, V Re, x Vibrations in the range of variables considered here increase steady heat transfer rates. The extent of this increase is determined by the relative strength of the oscillation parameter V re, x compared to that of Gr x . A maximum increase in heat-transfer rate of 33 per cent has been recorded. A plot of 4 Nu x/V 1 2 Re,x against Gr 1 2 x/V Re,x reveals that the results of the present investigation lie in the steady combined forced and free convection regime and are in adequate agreement with the approximate analysis due to Acrivos.