AbstractA technique has been developed by which molecular weight averages and distributions may be obtained for polymer fractions of high molecular weight.For glassy amorphous polymers, very dilute solutions in solvent‐precipitant mixtures are atomised on to a thin carbon substrate, shadowed with platinum‐carbon mixture and examined in the electron microscope. A solvent is chosen which is more volatile than the precipitant, so that on the path from the atomiser system to the substrate, and on the substrate itself, preferential evaporation of the solvent takes place. Contacts between polymer segments become more favourable than polymer‐solvent contacts, resulting in the progressive contraction of the molecular coils, and the deposition of single spherical self‐supporting molecules, the diameters of which can be determined from the lengths of the shadows cast by them.Molecules of polymers having glass transition temperatures below room temperature, or even slightly above, show varying degrees of collapse when prepared at room temperature. A device has been constructed which allows such polymer molecules to be deposited and shadowed at temperatures down to −100°, at which temperature molecular collapse is not apparent. Molecular diameters may then be measured as for glassy polymers. An atomiser‐separator system was constructed, which produced small droplets containing, essentially, one or zero polymer molecules, when a fraction of high molecular weight was used. All the solvent was evaporated from the droplets on the path to the cooled substrate, allowing the deposition of solvent‐free molecules.Molecular weight averages and distributions are presented for a series of high‐molecular‐weight polymer fractions, in the glass transition temperature range from 100° to —73°. The high precision of the results from this technique is demonstrated, together with the close agreement between molecular weight averages obtained by electron microscopy, and those obtained by light‐s