Simultaneous measurements of the size distribution were performed for fine particulate matter (diameter d < 0.5 μm) at the inlet and outlet of the electrostatic precipitators (ESP) of two full-scale pulverized coal power stations (615 MW, 510 MW). For a comparative study of the performance of the ESP the same high resistivity coal was burned at both sites. In addition, measurements were carried out for an easy to handle coal at the newer state-of-the-art power station. Effects on the size distribution caused by nonintermittent pulse energization of the ESP were also investigated. The results revealed a significantly stronger influence of the boiler than of the coal type on the size distribution at the inlet of the ESPs. In all cases the distribution was unimodal and a pronounced peak could be observed around 100 nm particle diameter. The ESP outlet distributions varied much more and showed also a dependence on the coal type. The maximum of the penetration through the ESP was determined in the range from 300 to 400 nm for all configurations. At the newer power station the number of ultrafine particles (< 30 nm) at the ESP outlet exceeded the inlet concentration under certain conditions. This effect was strongly influenced by the ESP energization and seemed to be related to the denitrification unit (DeNOx) installed upstream of the ESP where ammonia is injected as reducing agent. We conclude that ultrafine particles are formed inside the ESP and suggest that the ammonia injection has a decisive influence on their formation.