The results of an extensive study of the relative velocity of two-phase mixtures at ANL are presented. The parameter ranges studied are pressure 150–2000 psi, mixture quality 0–0.25, superficial liquid velocity 0.5–8 ft/sec, and flow channel equivalent diameters of 0.4–2 in. The data were correlated by means of the velocity ratio (steam velocity/liquid velocity) which was calculated from the measured steam volume fraction. The steam volume fraction measurements were made by a radiation attenuation technique and the data were obtained from both adiabatic and nonadiabatic systems. The data show that the velocity ratio is affected primarily by pressure, mixture quality, superficial velocity, and to a lesser degree by the flow channel geometry. The data are also compared with the data of other investigators for the vertical up-flow of steam-water mixtures. Working curves for the prediction of the velocity ratio are then presented which are adequate for system analyses. The working curves are given for 150, 250, 400, and 600 psi. A method of extrapolating the data for predicting working curves in the high-pressure range is suggested.