The self-accelerating decomposition temperature (SADT) is according to the United Nations Recommendations on the Transport of Dangerous Goods needed for transport classification of goods as class 4.1 (self-reactive). One of the methods recommended to determine the SADT is the Dewar-method. The method is well suited to assess the transport stability of liquids, whereas for solids the assessment errs on the unsafe side. The differences in heat transfer between solids and liquids are explained using cooling curves and classifying them by their Biot and Fourier-numbers and by applying the theories of Semenov and Frank-Kamenetskii for the critical heat release rates. (C) 2002 Elsevier Science B.V. All rights reserved.