A density of states Monte Carlo formalism is used to calculate the interfacial tension of a simple fluid (3D Ising model) and of free-standing polymer films on a lattice. Good agreement is found between the results of these calculations and literature values. The proposed approach is efficient and provides the surface tension over a wide range of temperature from a single simulation. It has the additional advantage of being equally applicable to systems interacting through continuous potential energy functions, discontinuous functions, and systems on a lattice. (C) 2003 American Institute of Physics.