The development of microstructural and electrical characteristics (sheet resistivities, TCRs, and noise indices) in some thick-film resistors during the firing process has been evaluated. Three 1 kohm/sq. resistor pastes (Du Pont), based on RuO2, ruthenate or a mixture of both conductive phases, were fired at temperatures from 500degreesC to 950degreesC. The cell parameters of the RuO2 in the 8031 and the 2031 resistors, the bismuth ruthenate in the 8029 resistors and the lead ruthenate in the 2031 resistors, were calculated from X-ray data. Microstructures were analyzed by SEM and EDS microanalysis. The absolute values of the cold and hot TCRs first decreased to a minimum at 850degreesC and then increased again with increasing firing temperature. The noise indices of the resistors generally decrease with increasing firing temperature.