This paper describes the development of a constant volume, variable pressure permeation system for use in testing membrane materials up to pressures of 276 bar with both a vacuum and pressurized downstream. In addition, a membrane masking technique that is well-suited for use with this system in high-pressure applications is also described. To illustrate the utility of the equipment, permeation and selectivity results for a commercially available polyimide, Matrimid, under pure and mixed gas conditions are presented and discussed in relation to studies reported at lower feed pressures with more conventional systems.