The reaction between methane and the lattice oxygen of a NiO/MgO (1/1.85) solid solution was investigated by an isotopic pulse-GCMS method. During a single-pulse reaction of CH4/CD4 (1/1) with the lattice oxygen, besides CH4, CD4, carbon monoxide and water, very small amounts of CHD3, CH2D2 and CH3D were detected in the exit gas. Because the isotopic kinetic effect (K-H/K-D) was 1.01, the dissociation of CH4 cannot constitute the rate-determining step. The rate-determining step is the reaction with the lattice oxygen, because of the high stability of the lattice oxygen in the NiO/MgO solid solutions.