The reflection shift δ2Θ caused by a radial shift δr of the sample away from its tangential position at the focusing circle is examined for grazing incidence diffraction and grazing excidence diffraction. Experimental results for residual strain/stress evaluation on thin films using a Bragg–Brentano diffractometer with a grazing incidence equipment are presented. Grazing excidence diffraction is less sensitive to δr than grazing incidence diffraction.