Hydrolysis of mixtures consisting of sodium borohydride NaBH4 (SB) and ammonia borane NH3BH3 (AB) was studied in the absence/presence of a Co catalyst. The kinetics of the H-2 evolutions was measured. The reactions were followed in situ by solution-state B-11 NMR and the hydrolysis by-products characterized by NMR, XRD and IR. It is demonstrated that the combination of the two compounds gives a synergetic effect. SB rapidly reduces the Co catalyst precursor and the NH4+ ions from AB contribute in the dispersion of the in situ formed Co nanoparticles. As a result, the kinetics of H-2 evolution is greatly improved. For instance, a hydrogen generation rate of 29.6 L min(-1) g(-1) (Co) was found for a mixture consisting of 81 wt% NH3BH3, 9 wt% NaBH4 and 10 wt% CoCl2. By B-11 NMR, it was showed that the reaction mechanisms are quite trivial. As soon as the Co catalyst forms in situ, SB, rather than AB, hydrolyzes until it is totally converted. Then, the overall hydrolysis continues with that of AB. Both reactions follow a bimolecular Langmuir-Hinshelwood mechanism; no reaction intermediates were observed during the process. In fact, SB and AB convert directly into B(OH)(4)(-), which comes in equilibrium with a polyborate compound identified as B3O3(OH)(4)(-). All of these results are discussed herein.