Abstract An examination of the convergence and efficiency of general purpose chemical process simulators indicates that there is room for significant improvement through the use of efficient local thermodynamic approximations generated in the context of the specific problem by reference to a full thermodynamic computer package and data base (thermopac). Currently such thermopacs do not provide information on the functional form of the properties that they return to the process algorithms. The exact form of the functional surfaces is quite complicated in most cases. Thus, it is desirable to develop a scheme in which the surfaces can be approximated by a number of locally valid approximate models. A scheme has been implemented in the form of a Thermodynamic Interface Package. This is designed to increase the flow of information between any chemical plant simulation system and any thermopac. The package contains subroutines to set up local models of fugacities and enthalpies in a dynamic manner. It includes the logic necessary to decide on the correct local model to use at a particular point in the simulation and also the logic to maintain the number and accuracy of such models with respect to the rigorous thermopac for a specified level of accuracy.