The use of an x-ray tube that emits a spectrum of energy was shown to be an effective source of monoenergetic radiaton for the measurement of the void fraction in two-phase, steam-water flow with the proper selection of the tube wall thickness, x-ray tube operating conditions, collimation, and traverse time. Assuming monoenergetic radiation, equations were derived for the local and average void fraction and the statistical error associated with the measurement, and were confirmed to be applicable by using the mock-up technique. This consisted of simulating the two-phase, steam-water flow patterns of the experimental system with mock-ups of Lucite and air. Exprimental data are presented to show that confidence could be placed in the measurements to within the probable statistical error, as all deviations of the measured values of the void fraction from the actual values were less than the probable error. Local and average void fractions for steam-water, two-phase flow are given. Application to other systems can be inferred from these results.