Physical and chemical properties of pure compounds and mixtures are essential for the analysis and design of chemical processing systems. A method for the estimation of properties of organic compounds from their molecular structure is presented, based on the contributions of Atoms and Bonds in the properties of Conjugate forms of a molecular structure (ABC). A real chemical compound can be considered the hybrid of a number of conjugates, which are alternative formal arrangements of the valence electrons of the molecule. The property-estimation method generates all conjugate forms of the molecule and assigns properties to each conjugate, simply by summing contributions from atoms and bonds in the particular electronic arrangement of the conjugate. The properties of the actual compound are then derived from the properties of the conjugates. The generation and analysis of conjugates is based on symbolic computation and Object-Oriented Programming (OOP). Atoms, bonds, molecules, electron pairs, and other entities can be represented as interconnected objects within OOP. The generation, comparison, and analysis of conjugates can be carried out through computer- based manipulation of the objects and their interconnections. One needs to encode operators which generate the conjugates, as well a rules for pruning the generation so that only the most important conjugates are considered. The nature and connectivity of atoms within a molecule determine the physical and chemical properties of the molecule. Traditional group-contribution methods eliminate much of the detailed molecular-structure information at an early stage of the property-estimation effort. Through symbolic computation and the concept of conjugation, the ABC approach aims to use molecular structure information more effectively.