A resistor system consisting of bismuth ruthenate and glass frits is used to study the effect of glass frits on the microstructure development and electrical properties of resistors which have different compositions and different softening points. As the glass content increases, the sheet resistance of the resistor films increases, which is significant in glass of low softening point. When the firing temperature is varied from 650-degrees-C to 950-degrees-C, the resistance decreases with increasing temperature. The temperature dependence of the resistance turns from negative to positive values with increasing firing temperature. Finally, it is shown that the microstructure of the film influences the electrical properties of the thick film.