This study describes an experimental investigation of the sealing performance of a metallic conical poppet and a polymeric annular seat with a round sealing edge. The experiments were carried out with nitrogen. The influence of the radius of the sealing edge and its surface roughness, on the sealing performance, is evaluated. Generally, it was found that for a given load, as the radius of the edge decreases, the maximum contact pressure increases, and the sealing performance improves. In addition, a lower surface roughness yielded better sealing for a given load. Surprisingly, a range of radii was identified where a smaller radius would have worse sealing compared to a larger radius under the same maximum contact pressure. It is suggested that this is due to a secondary effect of the sealing width on the sealing performance. It was shown that the contact width is influential only for relatively low loads, while for higher loads, the main contributors are the maximum contact pressure and surface roughness. This can imply that for a limited range of compressive loads, an optimal radius of the sealing edge exists, for best sealing performance.