Diabatic distillation, i.e. with heat exchangers on all trays, can increase the second law energy efficiency drastically compared to adiabatic distillation. The purpose of this research was to describe the entropy production rate in one adiabatic and one diabatic experimental water/ethanol rectifying column by applying the theory of irreversible thermodynamics. As a benchmark for the description, an exergy analysis of the two columns was used. This analysis showed that the diabatic column looses 39% less exergy than the adiabatic column. Heat and mass transfer on the trays and in the heat exchangers determined the entropy production rate, and neither pressure drop nor mixing effects played a large role in these columns. The significance of this work is the experimental confirmation of the theory on diabatic distillation. (C) 2003 Elsevier Science Ltd. All rights reserved.