The authors disclosed by the "simultaneous Q-DTA, Q-TG" method a latent, till now not known error in determining the decomposition heat of salt hydrates decomposing congruently or incongruently. This error cannot be shown by the traditional calorimetric or thermoanalytical methods owing to overlapping of the processes taking part in the heat decomposition, it can only be detected and eliminated by applying the "simultaneous Q-DTA, Q-TG" method of very high resolution and selectivity. A calculation technique is elaborated for the elimination of this error. (C) 2005 Elsevier B.V. All rights reserved.