Abstract In order to raise the degree of reduction and promote the formation of zerovalent cobalt clusters, repetitive treatments of CoNaY with hydrogen and oxygen were carried out. The intensity of FMR (ferromagnetic resonance) band of CoNaY reduced for 36 h at 773 K with intermittent oxygen treatments was twice as much as that of CoNaY reduced only with hydrogen. This fact meant that reduction of CoNaY with H2–O2 treatments favored to produce zerovalent cluster due to the increase in the mobility of Co2+ ions in CoNaY. The size of clusters is so small that such conventional methods as hydrogen chemisorption, X-ray diffraction, and TEM can not detect the quantity of zerovalent cobalt clusters. FMR spectroscopy, H2-consumption measurement, and catalytic probe reaction method were used to measure the degree of reduction and were correlated well with one another.