Abstract Typically, wavelength instabilities at synchrotron beams are largely due to thermomechanical phenomena at the primary-beam monochromators and fluctuations of the synchrotron orbit. Although they are small, they may have a negative influence on some kinds of diffraction experiments, for example in thermal expansion studies. Using a wavelength calibration procedure is a natural way of solving or at least reducing this problem. Diamond can be used as a calibration material because of its known low-thermal expansion and low X-ray absorption. Low-temperature X-ray powder diffraction measurements were carried out at the powder diffractometer at the B2 beamline at Hasylab/DESY (Hamburg). The cryostat ensured a good temperature stability and accuracy. Unit-cell parameters for cubic (spinel-type) silicon nitride, c-Si 3N 4, and for copper indium selenide, CuInSe 2, were determined in the temperature range from 14 up to 302 K. An improvement of the data quality due to elimination of the wavelength fluctuation effect is demonstrated.