Although it has been known empirically for some considerable time that the area under the peak of a thermogram (i.e. the area BCD on Figure 1) is approximately proportional to the amount of reactive constituent in the sample, only some five years ago was a simplified theory accounting for such a relationship proposed. This theory is, however, at best an approximation, and recently Arens has attempted a more rigorous mathematical treatment of some of the problems involved. Some incorrect assumptions and derivations in his treatment unfortunately rather vitiate the final result, and, in view of the wide interest likely to be exhibited in this theory, it is perhaps not out of place to draw attention to the inaccuracies here.