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This paper describes a technique for the quantitative analysis of nonradioactive iodine. The method uses the X-ray absorption discontinuities for the detection of iodine in the presence of any other absorbing substance. A stationary lanthanum radiator converts an X-ray beam having a continuous spectrum into two monochromatic beams which straddle the iodine absorption edge. The energy spread is such as to make the technique relatively independent of other foreign elements. A rotating filter alternately absorbs one of the two beams while leaving the other on continuously. The information in the beams is detected and synchronously switched into two counting circuits. The intensity of the beams is measured by two counting-rate meters and the ratio of these intensities is obtained by a conventional self-balancing potentiometer. Since this ratio is determined every 1/60 of a second the iodine determination is insensitive to X-ray output, amplifier and detector drift.