We developed a method to test totally encapsulating chemical protective (TECP) suit vent valves. The work reported here is a preliminary investigation of four small valves. Two of the valves were low-pressure vent valves from suits and two others were respirator valves. The latter were a pressure demand type and a standard flapper valve. For testing purposes, a single valve was mounted within a plastic plate. The plate was then installed between two halves of a testing box and consequently served to divide the box into two compartments. Each valve functioned as the only conduit between the separate compartments. We observed that methane gas penetrated through one of the TECP vent valves and the pressure demand respirator valve when each was in a “closed” position (with zero differential pressure across the valve). We also observed that when air pressure on the “inside” of each valve was increased, the leak rate decreased. This effect occurred under both static and dynamic conditions. The dynamic conditions were achieved with the use of a variable rate breathing machine. Further research is needed to allow more general conclusions to be made.