A Philips E.M. 300 electron microscope operating in the dark field high resolution mode has been used to study the size dependence of the melting points of individual particles of indium. The straight line graph, obtained from a simple thermodynamic treatment of the results, shows that the surface tension at the interface between solid and liquid indium is σ1 = 101 ± 10 dyn/cm. Similar experiments were carried out on tin so that the results of the present experiments could be compared with those of other workers. The value of the interfacial surface tension was σ1 = 69 ± 6 dyn/cm.