The aim of this work was to investigate the collection efficiency of several popular ionization chambers used in high-energy electron and photon dosimetry. The recombination effect was evaluated in plane-parallel type ion chambers (Markus, NACP, Calcam and Vinten-631) and in a thimble-type chamber (NE2571). The chambers' response was studied in a pulsed accelerator beam, for both photons and electrons, and in continuous radiation from a 60Co machine. The study involved both theoretical and experimental evaluation of the recombination effect utilizing the methods described in the literature. The main conclusions to be drawn from this work are as follows: (i) For the Farmer NE2571 0.6 cm3 chamber used here, the correction factors required are of the order of 0.1% for 60Co radiation, 0.5% for 5 and 8 MV x-rays and 10 and 12 MeV electrons and 0.9% for 15 MV x-rays and 15 and 18 MeV electrons, all at typical clinical dose rates of 200-400 cGy min-1 (0.017 cGy pulse-1). (ii) The recombination effect for the plane-parallel chambers was found to be smaller than for the thimble chamber, with values of < 0.1% in continuous radiation and 0.2-0.4% in pulsed radiation.