Pulse reactions of CH4, O-2 and mixed gas (CH4/O-2 = 2) over fresh, partially reduced and well reduced Ni/theta-Al2O3 catalysts have been carried out at 923 and 1023 K to study the mechanism of CH4 activation and the catalytic partial oxidation of CH4 The fresh catalyst showed high activity for the dissociation of CH4, and CO rather than CO2 was the main carbon-containing gaseous product while H, being detected for the first CH4 pulse, suggesting that CH4 can be partially oxidized to CO and H-2 by the lattice oxygen of NiO. The bond strength of NiO seems to be favorable to the partial oxidation of CH4, which is due to the strong interaction between NiO and theta-Al2O3. The results of CH4/O-2 pulses indicate that both metallic nickel and NiOx (x < 1.0) are active phases for partial oxidation of CH4. It has been demonstrated that both the reaction temperature and the C/O ratio are critical to the selectivities to CO and CO2. The mechanisms of the oxidation of CH4 over fresh and reduced catalysts have been proposed. (C) 2002 Elsevier Science B.V. All rights reserved.