Transmission electron microscopy (TEM) has been used to determine the porosity of polymer nanofoams produced by thermal decomposition of the thermally labile component of different copolymers. The results art compared with those obtainable by i.r. spectroscopy and density gradient methods, showing quantitative agreement for small pore sizes. TEM permits much higher spatial resolution than i.r. spectroscopy and is better suited to structures which scatter strongly at i.r. wavelengths.