Ion densities in a closed laboratory due to d.c. corona from an impulse-voltage generator and its charging set have been measured by means of a Gerdien-type aspirator. By comparison with natural ionisation, the densities of ions of the same polarity as the generator charging voltage were increased by two orders of magnitude, but ions of opposite sign were greatly reduced in density. Use of a filtering aspirator yielded information on the densities of ‘large’ ions which were shown to be an important part of the total ionisation occurring under natural and corona conditions. The significance of the results for impulse-voltage testing is briefly discussed.