The recent paper by Jankovic et al. [B. Jankovi, B. Adnadevic, J. Jovanovic; Thermochimica Acta 452 (2007) 106] and other similar papers have raised a problem which needs to be resolved. These authors use the method of Kennedy and Clark [J.A. Kennedy, S.M. Clark. Thermochimica Acta 307 (1997) 27] which is conceptually erroneous; this is analyzed in this paper. (C) 2009 Elsevier B.V. All rights reserved.