In this experimental study, the design parameters and performances of counterflow vortex tubes are investigated. Under different inlet pressures, the thermal performance as a function of the following geometrical parameters is studied: the length of the vortex tube, the diameter of the inlet nozzle and the angle of the control valve. Three different working gases are comparatively tested: air, oxygen and nitrogen. Temperatures, pressures and mass flow rates for the inlet and hot/cold exits, and temperature distributions at the wall are measured. On the optimum geometry, flow visualizations are also conducted in order to have more information about the flow inside the tube. It is disclosed that the inlet pressure and the cold fraction are the important parameters influencing the performance. (c) 2005 Elsevier Ltd. All rights reserved.