The gas separation taking place in the vortex tube is studied in detail. Both enrichment and depletion of a given component in any one of the two resultant streams may take place; the sign of this separation effect depends on certain parameters, notably the hot to cold flow ratio. A comparison of the data shows how the pattern of the effect curve, i.e. the separation effect as a function of hot flow fraction, varies with constructional parameters. Among these the ratio of the diameters of the two orifices through which the gas escapes from the tube, is of paramount importance. Also their magnitude relative to the tube diameter has a distinct modifying effect. The separation ability as a function of the tube length has a maximum at quite short lengths, dependent, however, on the inlet jet diameter in such a way that an increase in this causes an increase in the optimal length. The conclusion is reached that the centrifugation of the air, and only that, creates the gas separation detected in the outgoing streams. Its relation to the well-known temperature difference also produced between the two streams, is discussed. A flow scheme involving the radial and axial flow components, which permit an interpretation and a correlation of the experimental data in a simple way, is put forward.