We describe the set-up of an energy-dispersive reflectometer installed at the 6 T wavelength shifter (WLS) of the storage ring BESSY-I. It can be used for reflectometry and in-plane diffraction studies of thin films under ambient conditions. Due to the exponential decay of the incident spectrum, the whole useful energy range between keV is not available simultaneously. Absorbing the highly-intensive low-energy part of the spectrum, the specular reflectivity can be recorded up to using maximum angles of incidence and exit of . Up to the intensity is high enough to detect small angle Bragg peaks of a fatty acid salt multilayer separated by , (d is the multilayer period) in about 15 s with sufficient counting statistics. After normalization by the incident spectrum the energy-dispersive reflectivity spectrum provides information similar to that from an angular-dispersive experiment, but in a much shorter time. Besides the specular reflectivity, our set-up provides the possibility of simultaneous measurement of the in-plane momentum transfer (grazing incidence diffraction). For in-plane lattice parameter measurements a second detector is attached on a circle perpendicular to the plane of incidence. As in the angular-dispersive set-up, it enables investigation of the in-plane arrangement of a fatty acid salt multilayer in about 15 s. Keeping the in-plane Bragg angle fixed, the respective in-plane Bragg peaks appear for different energies and can be selected by changing .