A method and a device are disclosed for segmenting at least one substance in an x-ray image. In at least one embodiment of the method, two attenuation values relating to different energies of an x-radiation are acquired in relation to each pixel. The pixels are mapped into a two-dimensional feature space on the basis of the acquired attenuation values, and there is subsequently determined as assessment variable from the mapping of an image environment respectively assigned to the pixels the number of those pixels that fall in a classification region assigned to the substance. It is then possible to calculate in a simple way on the basis of the assessment variable a segmented image that contains the substance and in which misclassification can largely be avoided by taking account of the neighborhood relationship of the pixels.