Pumping and leakage of gas in radial lip seal were studied using a radial seal tester. Gas moved in and out across the lip, and the gas pump rate was generally larger than the gas leakage rate. Measurement of the solubility of gas in oil and the generation of cavities in oil indicated that the transportation of gas across the lip depended more on the cavity formation than the solubility. It was concluded that gas moved mainly in the form of cavities entrained in the pumping flow of oil.