This paper reports new data on the production of hydrogen from water vapor plus NaBH4, or NaBH4 + 10% CoCl2. Data were collected with the aid of an isothermal semi-batch reactor with in-situ H-2 rate measurement. The reaction of NaBH4 to generate H-2 proceeds via three steps: deliquescence, dissolution and reaction. The deliquescence regime of NaBH4 in the presence of 10 weight percent CoCl2 is defined. The H-2 yield is quantified at various reaction conditions (reaction temperature 70-120 degrees C, relative humidity 31-69%). CoCl2 significantly accelerates the rate of H-2 production compared to deliquescence + reaction of pure NaBH4. It is also found that a combination of high temperature and high relative humidity contributes to high H-2 rate and yield, and either of the two factors dominates the reaction at different conditions. A two-part reactor model accounting for the mechanism of the steam hydrolysis by NaBH4 is developed. The model captures the dissolution + reaction step as well as reaction-only step and was validated by experimental data. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.