The addition of NaBH4 to Co-ethylenediaminetetraacetate (Co-EDTA) and Co-citrate solutions at 25 degrees C does not lead to generation of hydrogen. However, in the presence of Co-based catalysts synthesized via chemical reduction of Co-EDTA and Co-citrate complexes with NaBH4 at elevated temperature, an intensive generation of H-2 took place. In this study, the reduction mechanism of both complexes was elucidated by using various techniques. From the results of attenuated total reflection and mass spectrometry analysis, it was suggested that NaBH4 was oxidized to NaBO2 and that organic ligands of Co complexes were decomposed to gaseous hydrocarbons, such as C2H4, C3H4, and/or C2H3N. Structural characterizations of X-ray diffraction, scanning transmission electron microscopy, transmission electron microscope, energy-dispersive spectroscopy, and X-ray photoelectron spectra on the catalysts revealed that Co(OH)(2), metallic cobalt, and cobalt borate were obtained in both cases. The morphology of Co(OH)(2) and the dispersion of metallic cobalt and cobalt borate nanoparticles were significantly different. In the case of the catalyst prepared from Co-EDTA, the nanoparticles of Co species aggregated with diameters from 100 to several hundred nanometers on Co(OH)(2) slabs. On the catalyst prepared from Co-citrate, the Co(OH)(2) formed sheets, and the nanoparticles of Co species formed clusters of 5-10nm in diameter, which are dispersed well on the Co(OH)(2) sheet. The catalyst obtained from Co-citrate showed higher catalytic activity on hydrolysis reaction of NaBH4 than that from Co-EDTA. Copyright (c) 2016 John Wiley & Sons, Ltd.