Abstract: In a connecting device for connecting chromatographic separating columns of glass to terminal fittings on a chromatographic apparatus, each end of the column is surrounded by a metallic sleeve. A sleeve made of synthetic plastic material, which is of substantially stable shape at least up to a temperature of 350.degree.C. even if subjected to pressure, is interposed between said end and said metallic sleeve. A preferred material is polyimide. The plastic sleeve has at least one conical end face and is held in sealing engagement with the end of the separating column, on one hand, and with the metallic sleeve, on the other hand, by a sleeve shaped thrust member, which is axially movable relative to the metallic sleeve to exert an axial force on said plastic sleeve. A radial sealing force is exerted on the synthetic plastic sleeve through its conical end face by an abutting complementary conical surface.