True-boiling-point distillation is an efficient batch distillation being one of the most commonly applied techniques to study the polydispersity of complex hydrocarbon mixtures. True-boiling-point curves are obtained by plotting the temperature at the top of the column versus the fraction of mass distilled. The name expresses the assumption that the complex mixture is separated into its components according to their boiling points. However, in the case of azeotropy (e.g. between aliphatics and aromatics) this assumption is not correct. Therefore, based on continuous thermodynamics, a method is developed for describing quantitatively the azeotropic effect. The method presumes distillation of pure compounds or of azeotropic mixtures (if the efficiency of the column is sufficiently large). It is demonstrated for mixtures consisting of a petroleum fraction (low in aromatics) and of alcohols.