Cobalt(II,III) oxide nanopowders are used as supporting materials for rhodium(0) nanoparticles forming Rh-0/Co3O4 nanocatalysts, which can be prepared by impregnation and sodium borohydride reduction of Rh3+ ions on the surface of the oxide support. Magnetically separable Rh-0/Co3O4 nanoparticles are isolated from the reaction medium by an external magnet and characterized using various analytical techniques. Rh-0/Co3O4 nanoparticles are highly active and reusable catalysts with a long lifetime in hydrolytic dehydrogenation of ammonia borane (AB) at room temperature. Rh-0/Co3O4 nanoparticles with 0.5% wt Rh loading provide a turnover frequency of 1800 min(-1) and a total of 1.02 x 10(6) turnovers for H-2 evolution from the hydrolysis of AB at 25.0 +/- 0.1 degrees C. This turnover frequency is the second best value ever reported for the hydrolysis of AB, and the total turnover number of over a million is a record lifetime ever reported. Magnetically separable rhodium(0) nanoparticles are expectedly highly reusable catalysts and preserve their initial activity after the fifth run of hydrolysis. We also report the results of our study on the catalytic activity of Co3O4 nanopowders for the same dehydrogenation reaction.