Mathematical model equations and their corresponding solution methods for membrane gas permeation and pervaporation were developed for direct use in process simulators, without the need for external, custom programming. The models are based on the solution-diffusion mechanisms for species permeation through nonporous membranes. Cross-flow and counter flow models using the logarithmic-mean trans-membrane partial pressure were used to simulate spiral wound and hollow fiber membrane configurations. The Chen approximation of the log-mean partial is recommended for avoiding divergence in the iterative solution methods required to solve the non-linear model equations. The models incorporate temperature, pressure, composition, mass flow, membrane area, and species permeance effects into the simulations. The models were designed for implementation in process simulators using intrinsic capabilities for calculating material and energy balances and predicting physical and thermodynamic properties. Examples are given using the process simulator HYSYS.