Abstract The energy distributions of ions in different charge states extracted from an ECR ion source were analysed using a double hemispherical energy analyser. The effect of the finite ion temperature can be separated from other mechanisms broadening the energy distribution. It is found that the ion temperature has only a small influence on the energetic width of the extracted ion beams. Additional broadening is enhanced by large extraction gaps and a large fieldstrength in the extraction gap. Pressure inside the source and applied microwave power turn out to be of negligible influence.