An improved method and apparatus are disclosed for isolating product gas from compressor operating fluid. A portion of a piston rod directly exposed to the compressor's operating fluid (the oily portion of the piston rod) is atmospherically isolated from the remaining portion of the piston rod by providing the piston rod with a collar that separates the oily portion of the piston rod from the remainder of the piston rod. A first and a second flexible membrane are coupled to the collar to form a first isolation region, and a second isolation region; the first isolation region encases the oily portion of the piston rod and atmospherically isolates it from the second isolation region, and the second isolation region provides additional isolation from both the operating fluid and the ambient environment. A higher pressure is maintained along the backside of the piston that compresses the product gas than the pressure within the adjacent isolation region. Pairs of wipers with vents coupled therebetween additionally may be employed to further improve product gas isolation.