AbstractTrouton's rule is modified to allow for the fact that the entropy of vaporization is generally a function of the Guldberg ratio and the parachor‐critical volume ratio. The medification has the advantage that it is applicable to non‐associated substances of low boiling‐point, and furnishes a rational basis for their classification into normal and abnormal liquids. Moreover, a high value of the medified ratio is diagnostic of association in the liquid