Standard flanges for five widely differing pressure ratings, having a wide range of different joint surface profiles, were sealed by flat rubber or asbestos gaskets. Different initial bolt tensions were applied and the variation of clamping force with internal pressure was measured up to leakage of the joint. The joint efficiency, defined as: (end thrust due to leakage pressure on bore area of pipe)/(total initial bolting force), is not affected by variations in joint-face surface finish if machining grooves across the joint surface are avoided. Minimum values of joint efficiency are given. The effects of gasket material, width and thickness and number of bolts on joint efficiency are considered.