Similarities and differences in the thermal behaviour of deuterated and nondeuterated ammonium perchlorate provide insights into the mechanism of its thermal decomposition. Thermal decomposition of NH4ClO4 and ND4ClO4 always begins in the bulk of the crystals. In both cases decomposition stops when the degree of conversion is about 30%, giving porous products which undergo the same phase transition as the parent single crystals. Thermal decomposition of the deuterated sample is slower, the volume fraction of pores appears to be lower and the sample has a small quantity of 'snow' on the surface. These effects are best rationalized as caused by proton transfer at the intersections of dislocations in the bulk of the crystals. (c) 2008 Elsevier B. V. All rights reserved.