Abstract The polarized X-ray absorption cross section of the ls → 3d transition in a square planar CuCl2−4 complex has been measured with respect to rotation about an axis normal to the CuCl4. The cross section exhibits four-fold periodicity about this axis, indicating that the transition is primarily due to coupling with the electric quadrupole component of the radiation. The vibronically allowed dipole transition is approximately one-third as the quadrupolar cross section. These observations are in agreement with SCF Xα multiple-scattered wave calculations, and may have implications for the intepretation of other X-ray absorption spectra. The half-filled d orbital is shown to have the angular characteristics of dx2−y2.