The extraction of ion beams from ECR ion sources is investigated. It is shown that for high magnetic fields within the extraction region, as in the CAPRICE‐type ECR, all different charge states have to be simulated simultaneously to achieve the correct space charge distribution of the extracted ion beam. The change of the longitudinal velocity distribution for different charge states is taken into account. Beam emittance measurements made 280 mm behind the extraction system will be compared with the results of the simulation. Two possible extraction systems which should have influence on the beam characteristics are investigated.