This paper reports net concentrations of small ions (NCSI) monitored at 41 sites under overhead high voltage ac power lines in and around an urban environment. The net ionic polarity under power lines was of both signs but mostly positive and NCSI varied widely from 0 to 3300cm−3. Concomitant measurements of the vertical dc electric field at the ground confirmed the presence of a net positive charge above. Approximately, 19% of the sites exhibited relatively high NCSI exceeding 1000cm−3. The mean value of all the sites was 776cm−3. Statistically, the mean for the transmission voltage (220–330kV) line sites was significantly higher than that for the sub-transmission voltage (110–132kV) line sites with means of 905 and 501cm−3, respectively. These values were compared with the mean urban outdoor concentration well away from the lines which was about 400cm−3 and of negative polarity. Overall, NCSI at approximately 76% of the power line sites exceeded the absolute mean urban outdoor value. The dc electric fields under the power lines showed a statistically significant relationship to the measured NCSI, although there was considerable scatter to indicate that electric field measurements do not necessarily reflect NCSI in the air at ground level.