Nano-sized nickel ferrite powders have been synthesized from fly ash via a chemical synthesis route, co-precipitation method. X-ray diffraction analyses showed that pure crystalline nickel ferrite, NiFe,04, phase can be obtained by thermal treatment of the precursors at temperature >800 degrees C for 120 min in the studied pH range, from 7 (neutral) to 12 (highly alkaline). In the temperature range of 500 degrees C <= T <= 800 degrees C, impure low crystalline NiFe2O4 phase is formed. The main impurities are FeO(OH) and Fe(2)O3 center dot H2O phases. Higher magnetization (32 emu g(-1)) is obtained for a precursor precipitated at pH 10 and thermally treated at 1200 degrees C for 120 min. Prepared nickel ferrite samples showed a good response toward CO oxidation. It is concluded that the lower crystal size (similar to 8.5 nm) enhanced CO adsorption and consequently its oxidation. (c) 2005 Elsevier B.V. All rights reserved.