We have found that a Ni-Cu-MgO catalyst maintained its activity for the decomposition of methane at high levels for substantially long periods of time at 665-725 degreesC, being capable of generating large amounts of CO-free H-2 and solid carbon. TEM examinations revealed that the solid carbon consisted exclusively of nanofibers possessing a "platelet" structure, in which the graphite layers are aligned in a direction perpendicular to the axis of the fiber. In sharp contrast, the Ni-MgO-methane system exhibited negligible activity at temperatures greater than or equal to650 degreesC. XRD and H-2 chemisorption characterizations indicated that after reduction a Ni-Cu alloy was formed in the Ni-Cu-MgO catalyst and the surface of the particles was enriched in Cu-0. A possible explanation for the observed superiority of the Ni-Cu-MgO catalyst over that of the Ni-MgO system is presented.