One kohm/sq. thick-film NTC thermistors (4993, EMCA Remex) with high, non-linear and negative temperature coefficients of resistivity were fired at different temperatures. The development of the resistors' conductive phase and microstructure was investigated by X-ray diffraction analysis and by scanning electron microscopy. Sheet resistivities, beta factors and noise indices were measured as a function of the firing temperature. In the fired layers of the thermistors the X-ray analyses showed mainly spinel phase and RuO2, which is added to the thick-film NTC materials to decrease the specific resistance and to improve the stability and the current noise. Higher firing temperatures led to more densely sintered microstructures, to increased resistivities and to higher beta factors. The higher resistivities were attributed to the partial exchange of ions on the "B" sites of the spinel structure with aluminium ions.