Ionization detectors have been characterized using a modified theory which includes soot particle charge fraction functionality in addition to the generally accepted particle size and number density dependence. A sensitivity factor has been determined which, for a given detector, is influenced by both the fire combustion mode and fuel source. This factor decreases as the fuel bond unsaturation and aromaticity increases and can be the soot yield for flaming fires. An order of magnitude increase in sensitivity factor for wood accompanies the transition from non-flaming to flaming combustion mode and suggests a large charge fraction in non-flaming wood smoke aerosols. This result is similar to that found in flaming aromatics such as polystyrene, and is consistent with the general ionization detector experience showing insensitivity to non-flaming fires.