Under certain conditions of electron temperature and mean free path the attenuation of an electromagnetic wave incident on a vacuum‐plasma boundary cannot be described in terms of the usual complex conductivity. In a cylindrical plasma with an electron density lying between 1010and 1012cm−3, and an electron temperature of 2.2 eV measurements of the attenuation of a small amplitude wave show resonance phenomena due to interaction between the wave and the moving electrons. A theory of this resonance for the case of a plasma slab is compared to the experimental results.