The entropy of vaporization at the normal boiling point (Delta S-b) is an important propel ty in the chemical engineering and environmental sciences. This paper presents a simple modification of Trouton's rule to estimate this property for a variety of hydrocarbon compounds including alkanes, alkenes, alkynes, dienes, cycloalkanes, and alkyl aromatics. The equation contains three easy-to-determine parameters that describe important molecular features: flexibility, symmetry, and planarity. The entropy data were calculated from experimental enthalpy data of 477 different organic compounds at the normal boiling point. The average percentage error for the prediction is 1.30 by the proposed method, 2.79 by Trouton's rule, 1.62 by Kistiakowsky's method, 1.51 by Vetere's method, and 2.23 by the group contribution method of Ma and Zhao.