This paper describes the method and results of experiments on effects of seal operating conditions on gas transportation between the air side and the sump side of radial lip seals. A novel technique has been developed to measure sucked and leaked gas volume across lip seals quantitatively by using gas chromatography. Tests are conducted with helium and nitrogen under various shaft speeds, sump levels, shaft dynamic runouts, and sump pressures. The results show that lip seals pump gas from the air side into the sump side, and at the same time leak gas in the opposite direction, whereby the leak rates are lower than the pump rates. The rates of gas transportation depend on the shaft speed and the sump pressure, while they do not depend on the dynamic runout.