The role of column holdup on the performance of batch distillation has been dealt with by many researchers in the past. It was found that increasing holdup in some cases improved column performance and that the effect was in other cases detrimental. However, none of the studies were done systematically to explain either the positive or the negative effects of holdup. Here we examine the role of column holdup for binary mixtures in a systematic way and explain both effects on physical grounds. It is found that the characterization of the column separation task using 'the degree of difficulty of separation ' [Christensen and Jorgensen (1987) Chem. Engng J. 34, 57] measure and the evaluation of column performance using 'the minimum time' measure are very useful in explaining the positive and negative effects of column holdup. We use two different but fairly detailed dynamic batch distillation models. For the purpose of direct comparison with the previous work we retained the assumption of constant molar holdup (CMH) in one of the models, and replace this by a more realistic assumption of constant volume holdup (CVH) in the other model. The study allows identifying at the design stage an optimum holdup required to achieve the best performance for a given column configuration and separation task.