A boss for a pressure vessel with an outer reinforcing shell and an inner liner has a radially extending flange and a tubular neck projecting outwardly to provide a fluid communication port. The flange is embedded in and structurally integrated with the material of the inner liner during molding. The flange is divided by a conical annular groove into an outer skirt and an inner skirt. The inner skirt protrudes from the outer skirt and has a flattened end facing toward the vessel wall. The flattened end and/or the surfaces of the annular groove are textured, knurled or otherwise unevenly surfaced for gripping the liner material. A number of apertures extend from inside the groove to the opposite side of the flange. The liner material is molded on and in the groove of the flange and fills the apertures to form anchoring segments integral with the liner, extending through the flange to liner material on both sides.