In this study, quantitative models for monomer/water and the corresponding polymer/water interfaces were developed. The monomer/water interface was modeled within the framework proposed by Fu and Li (Chem. Eng. Sci. 44 (1989) 1519) for organic liquid/water interfaces. We took a similar approach with that which others used to simulate micelle formation of surfactants and block polymers in developing the model for the polymer/water interface. In this model, the Gibbs free energy of interfacial formation was decomposed into two components: transfer free energy of a polymer segment to the interface and mixing free energy within the interface. Interfacial tensions were then estimated using concentration gradient theory. The ratio of the number of total nearest neighbors of a site in a lattice within the interface to that located in each of the adjacent planes was found to be the important correlating factor. It was shown that this ratio for monomer/water interfaces and that for polymer/water interfaces are linearly related. The polymer in the interface is likely to be in the form of short polymer segments (2–4 monomer units) rather than dangling chain ends. The estimation of polymer/water interfacial tension from the models established in this work is in reasonable agreement with experimental observation.