Abstract: In a system for controlling electric power supplied to corona-generating electrodes in an electrostatic precipitator (10), an opacity-sensitive transducer (20) produces an output signal proportional to the opacity of the flue gas exiting from the precipitator (10). The signal from the transducer (20) is compared in comparators (304 and 305) with pre-set upper and lower limits defining a permissible opacity range for the flue gas. When the signal from the transducer (20) exceeds the pre-set upper limit or falls below the pre-set lower limit, automatic voltage controllers (40) are activated to control the power supplied to the corona-generating electrodes in order to restore the flue gas opacity to the permissible opacity range.