NiO-LnO(x) (Ln = lanthanide) catalysts (with Ni : Ln = 1 : 1) without prereduction show high activity/selectivity and very high productivity in the oxidative conversion of methane to CO and H-2. The catalysts are first activated in the initial reaction, which is started at 535-560-degrees-C, by the reduction of NiO and creation of active sites. The carbon deposition on the catalysts in the reaction, particularly for the NiO-Gd2O3, NiO-Tb4O7 and NiO-Dy2O3 catalysts, is quite fast but it has caused a little or no influence on the catalytic activity/selectivity. Pulse reaction of pure methane on NiO-Nd2O3 (at 600-degrees-C) shows involvement of lattice oxygen in the initial reaction and also reveals formation of carbon from CO on the catalyst reduced in the reaction.