The present invention relates to a method and a device for identifying unknown substances in an object. According to the state of the art a collimated X-ray beam is directed onto the object and X-ray energy scattered from the object is detected and measured. The measurement values detected are compared to known measurement values corresponding to specific substances or classes of substances to identify unknown substances in the object. The method and device further execute the following steps: Detect and measure a function of Rayleigh intensity at a number of angles in a predetermined angular range to determine the angular course of the function of Rayleigh intensity in the angular range, Determine an angle or angle interval within the angular range at which the function of Rayleigh intensity is essentially zero; and identify unknown substances in the object by comparing the angle or angle interval determined to known angles or angle intervals at which the function of Rayleigh intensity corresponding to specific substances or classes of substances is essentially zero.