The semiconductor-to-metal phase transitions in oxides of vanadium and titanium were used to prepare composite thermistors which exhibit both NTC and PTC resistance effects. Composites were prepared by mixing V//2O//3 and other oxides in various proportions in both crystalline and amorphous polymers. These composites showed unusually large PTC resistivity increases of up to 10 orders of magnitude. Accompanying NTC effects as large as 5 orders of magnitude were observed at the semiconductor-to-metal transitions in the filler materials. Low room temperature resistivities were observed for all composites. The resistance vs. temperature curve of NTC-PTC composites resembles a square well with three temperature regimes: semiconductor to metal to insulator.