Polymeric aminoborane (H2BNH2)(x) has been isolated during the thermal decomposition of solid borazane H3BNH3 at temperatures below 370 K. Polymeric aminoborane is a white noncrystalline solid, stable at room temperature and up to 380 K. In the temperature range of 380-500 K polymeric aminoborane undergoes a thermal decomposition, which was studied by differential scanning calorimetry (DSC), by thermogravimetry (TG) and by volumetric measurements. The solid residue was characterized by IR spectroscopy, by powder X-ray diffraction (XRD) analysis and by solid state B-11 NMR spectroscopy. The thermal decomposition of polymeric aminoborane (H2BNH2), is an exothermic process accompanied by evolution of nearly 1 mol hydrogen per mole H2BNH2 unit corresponding to the volumetric measurements. The detected mass loss of the solid phase indicates the formation of further gaseous products beside hydrogen. The final mass loss increases with rising heating rate, in contrast to the final amount of evolved hydrogen, which still remains constant. (c) 2004 Elsevier B.V. All rights reserved.