The vertical intensity distributions of X‐rays from a synchrotron source are measured by powder diffraction. The components polarized in the orbital plane and perpendicular to this plane are separated by measuring a reflection 90° to the vertical and horizontal planes, respectively. The measurements are made at three wavelengths between 0.65λcandλc, whereλcis the characteristic wavelength. In each case, the degree of linear polarization reaches a maximum of 90% at the center of the beam. The results agree closely with calculations which use machine optical param