In the present work we studied the effect of cobalt halides (CoF(2), CoCl(2), CoBr(2) and CoI(2); also denoted CoX(2)) on the thermal decomposition of ammonia borane NH(3)BH(3) (AB) over the range 25-220 degrees C. The reaction was followed by thermogravimetric analysis and differential scanning calorimetry. The gaseous products analyzed by gas chromatography and mass spectrometry, and the solid by-products identified by elemental analysis: powder X-ray diffraction, infrared and X-ray photoelectron spectroscopies. Compared to pristine AB, the presence of CoCl(2) and CoBr(2) reduces both the induction period of the AB decomposition and the content of unwanted borazine in the H(2) stream, whereas the presence of CoI(2) or CoF(2) has little or no effect, respectively. We propose that the positive effect of CoX(2) comes from their electronic and steric effects, and that CoCl(2) is the compound which shows the best properties relative to these effects. Herein, the roles of Co and X are discussed and a revised mechanism of the AB dehydrocoupling initiation is proposed. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.