Abstract: An oxygen lance assembly includes a lance body which can be coupled and decoupled from a head through the use of a plug and socket junction. The plug part of the junction can be located on the lance body and provided with a plurality of axially spaced cylindrical male sealing surfaces. These are adapted to mate with corresponding female sealing surfaces provided in the head. The sealing surfaces mate with a clearance and the required seal is achieved by mounting O-rings exclusively on each male sealing surface. This provides for convenient inspection of the O-rings. Damage to the O-rings caused by vibration is prevented by cylindrical guide surfaces having a close sliding fit which serve to keep the plug part coaxial in the socket. Alignment of the plug in the socket is achieved by increasing the radius of each cylindrical sealing surface the further it is from the top of the socket via frusto-conical guide surfaces.