Tritium permeabilities were determined at room temperature, 1.0 MPa (150 psia) tritium for three 23.4 cm diameter EPDM (ethylene-propylene-diene monomer) O-rings using a full-scale mock-up of the AL-SX shipping container seal geometry. The AL-SX container is being developed by Sandia National Laboratories for shipping tritium reservoirs. To determine the tritium permeation rate as a function of temperature, a 50.8 mm diameter EPDM O-ring was tested from room temperature to 150° C at a pressure of 1.0 MPa. Additional permeation measurements were made under the following test conditions: (1) deuterium and helium-4 at room temperature and a pressure of 1.0 MPa using the full-scale AL-SX fixture, (2) tritium from 0.1 MPa to 1.0 MPa at 142° C using the 50.8 mm fixture, and (3) deuterium from room temperature to 150° C at a pressure of 1.0 MPa using the 50.8 mm fixture. Multiple permeation runs using the three full-scale O-rings showed the average room temperature, 1.0 MPa steady state tritium permeation rate to be about 1 × 10−2 Pa-liter/sec (7.6 × 10−5 torr-liter/sec or 1 × 10−4 std cc/sec), well within the allowable limit of 7.1 × 10−2 Pa-liter/sec for tritium release from the AL-SX container. Based on the temperature dependence derived from tests on the 50.8 mm fixture, the permeation rate through the large O-ring at 1.0 MPa tritium, 150° C would be about 60% of the allowable limit. The tritium permeability was found to vary linearly with pressure within the range explored.