Abstract: The valve system for opposed piston engines essentially comprises a single poppet valve opening into the common combustion chamber between the two opposed pistons of each piston and cylinder pair. The engine for which the mechanism is adapted includes a rotating internal cylinder surrounding each piston pair, with a stationary outer cylinder or case surrounding the rotating cylinder. The valve is pivotally attached at one side or end thereof to the edge of the valve port of the rotating cylinder, and is actuated by an arm or arms having guides (rollers, etc.) at the distal end(s) thereof, which are captured in corresponding cam track(s) or channel(s) formed in the fixed outer cylinder or case of the engine. The cam track has a variable radius, with the valve arm(s) and guide(s) alternately lifting and lowering as the guide(s) travel(s) in the variable radius cam track(s), thereby closing and opening the valve.