The results of an investigation into the behaviour of thin-walled steel tubes filled with high strength concrete are presented. The main emphasis is placed on the level of ductility that can be achieved, considering the fact that neither high strength concrete nor thin steel tubes are individually able to absorb significant amounts of energy under cyclic loading. Results of 26 tests on specimens with a diameter of 152 mm and a wall thickness of 1.7 mm, filled with concrete of characteristic compressive strength ranging between 73 and 92 MPa, are reported. Load combinations on the specimens range from pure axial compression, through various combinations of axial load and bending, to pure bending. Three specimens were subjected to cyclic loading. Test results are compared with design models that are used in current code specifications. Key words: steel tubes, concrete, composite, beam-column, beam, column, experimental.