[en] Although plane-parallel ionization chambers have been in use for some time, there is still much to be learned about their performance characteristics. This work is concerned with the polarity effect in electron beams about which there is little published data. The investigations involved several popular ionization chambers; the PTW Markus chamber, the NACP chamber and its Calcam version, the Vinten-631 chamber, and the NE 2571 (Farmer-type) thimble chamber. The chambers were irradiated in electron beams of nominal energies between 4 MeV and 18 MeV. (author)