The structural and electrical properties of NiMn2O4 and Ni0.5CoxMn2.5-xO4 (where x = 0.5, 0.8 and 1.1) NTC thermistors have been investigated. The samples, prepared by conventional ceramic processing techniques, were calcinated at 900 degrees C for 2 h and then sintered at 1100 degrees C and 1200 degrees C for 5 h. The cubic spinel phase was observed by XRD analysis in the NiMn2O4 and Ni0.5Co0.8Mn1.7O4 samples sintered at 1100 degrees C for 5 h. The sintering at 1200 degrees C resulted in much denser microstructures with a larger grain size. The room-temperature electrical resistivity (rho(25)) and material constant (B) value of the NiMn2O4 sample sintered at 1100 degrees C were 7710 Omega cm and 3930 K, respectively. The electrical resistivity of the samples decreased significantly with the addition of Co3O4. The B-25/85 values of the Ni0.5CoxMn2.5-xO4 (where x = 0.5, 0.8 and 1.1) samples sintered at 1100 degrees C were found to be 3820 K, 3525 K and 3270 K, respectively.