This paper describes an experimental study of the long term sealing performance of nitrile rubber joints on a manned deep ocean submersible operating down to 6000 m depths. An initial formulation showing large compression set is discussed first. Then results from long term laboratory testing of a second formulation, aged in water for 10 years, are presented. Much lower compression set and higher sealing forces are observed. Based on these results a testing methodology for the prediction of the long term sealing performance of nitrile O-rings is proposed. Results from laboratory tests are compared with those measured on joints aged for up to 10 years in service.