Abstract: The invention relates to a valve seal mechanism for a rotatable valve assembly that provides a sealing function between a rotating first valve element and a fixed second valve element as used in a rotary cylinder valve engine. In one embodiment the seal mechansim comprises a substantially rigid sealing frame which surrounds and sealingly engages the periphery of the valve port of one of the cyclindrical valve elements and also sealingly engages a surface of the other cylindrical valve element. In another embodiment, the seal mechanism comprises a resiliently deflectable tubular element of variable diameter, the tubular element being mounted around a first valve element with an aperture of the tubular element being radially aligned with a valve port of the first valve element, the tubular element being biased radially outward of the first valve element.