The values of charge on individual particles exiting three different laboratory precipitators have been measured in an experimental apparatus containing a Millikan cell. Measurements were obtained for dioctylphthalate (DOP) droplets and fly ash particles at temperatures from 23 to 343 °C. At comparable voltages and currents for positive and negative coronas, the data show that the ratio of the values of negative‐to‐positive charge for radii in the range 0.6–1.3 &mgr;m increases from about 1 to 2 as the temperature increases from 37 to 343 °C. The predictions of a mathematical model of electrostatic precipitation which employs an ionic charging theory show good agreement with all the positive charging data, but show good agreement with the negative charging data only at temperatures below 37 °C. The differences in the measurements and the model predictions are consistent with the postulation of free electron charging at elevated temperatures in negative corona.