The 1-butyl-3-methylimidazolium halide ionic liquids are stable up to temperatures of 246(1)degrees C ([BMIm]CI), 260(1)degrees C ([BMIm]Br), and 238(1)degrees C ([BMIm]I). The thermal decomposition proceeds in thermogravimetric measurements with a total mass loss of 100%. Using evolved gas analysis (EGA) a complete wdegradation of [BMIm]X ionic liquids under formation of characteristic fragments CH3+, NHn+, C4Hn+, and CH3X+ (X=Cl, Br, I) has been observed. [BMIm]CI shows enantiotropic polymorphism with a phase transition temperature at 30(1) degrees C, and melts at 74(1) degrees C (Hr, = 18 +/- 0.5 kJ mol(-1)). Spontaneous re-crystallization and reversible phase transition have been found for cooling of the substance. [BMIm]Br melts at 78(1) degrees C (Delta H-fus = 29 +/- 0.5 kJ mol(-1)). The re-crystallization fails and thus a glassy solid is formed. The glass transition temperature is about -65 degrees C, the cold crystallization occurs between -30 and -20 degrees C. The application of both homogeneous and heterogeneous nucleation agents does not interfere the glassy state. [BM1m]I undergo solidification without crystallization. The melting effect for the amorphous substance arise at -70(5) degrees C with Delta H-fus, = 0.4 +/- 0.2 kJ mol(-1). (C) Crown Copyright 2013 Published by Elsevier B.V. All rights reserved.