The present study reports fundamental results about the spontaneous hydrolysis of sodium borohydride NaBH(4), which is a potential hydrogen storage material for small, portable applications. The reaction (without stabilizing agent or catalyst) was carried out at temperatures of 30-80 degrees C, initial NaBH(4) concentrations of 0.63-6.19 mol L(-1) (2.3-18.9 wt%), and for unbuffered solutions, which are harsher experimental conditions than those reported so far. The H(2) evolution and the subsequent pH variation were observed to determine the reaction kinetic parameters and characterize the hydrolysis intermediates, i.e. the hydroxyborates BH(4-x)(OH)(z)(-), by XRD, IR and (11)B NMR. It was found that: (i) the apparent activation energy of the reaction was 98 +/- 10 kJ mol(-1) and the reaction order versus the initial NaBH4 concentration was 0; (ii) all of the reactions BH(4)(-) -> BH(3)(OH)(-), BH(3)(OH)(-) -> BH(2)(OH)(2)(-), BH(2)(OH)(2)(-) -> BH(OH)(3)(-) -> and BH(OH)(3)(-) B(OH)(4)(-) took place simultaneously; (iii) only 25 mol% of B(OH)(4)(-) and 75 mol% of BH(4)(-) were found at 25% of conversion; (iii) the hydroxyborates are very short-lived intermediates and only traces of BH(3)(OH)(-) were detected. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.