A solid-state coordination reaction was adopted to prepare negative temperature coefficient ceramics. A mixed oxalate NiMn2(C2O4)(3)(.)6H(2)O, a coordination compound, was synthesized by milling a mixture of nickel acetate, manganese acetate, and oxalic acid for 5 h at room temperature. An ultrafine NiMn2O4 powder was obtained by calcining the mixed oxalate in air at 850 degrees C for 2 h. Ceramics with a relative density of more than 97% were achieved by sintering powder compacts at a temperature as low as 1050 degrees C for 5 h. The specific electrical resistivity P-25 degrees C and the thermal constant B-25 degrees/85 degrees C were 2174 Omega (.) cm and 3884 K, respectively. The drift of the resistivity after aging at 150 degrees C for 1000 h was 3.0%.