Abstract: A catheter instrument for use in the continuous measurement of blood gases in vivo by mass spectrometry comprises a flexible mylon tube internally coated with a material such as polyurethane which inhibits outgassing of water vapour from the nylon. A porous body is secured in one end of the tube and has a smoothly rounded projecting portion covered by a gas-permeable membrane. The membrane forms an integral part of a sheath having a further part which fits closely around the nylon tube, the sheath being of a non-thrombogenic polymeric material such as silicone rubber.