An experimental technique for the direct observation of imperfections in single crystals is described. It is based on the anomalous transmission effect of x rays observed in crystals of high perfection. The parallel beam method previously used only for the mapping of dislocations has been improved so that large area x‐ray topographs are recorded. It can now be applied to the detection of impurities, segregation, and precipitation effects in semiconductor materials. Microsegregation of oxygen in silicon, precipitation of copper in silicon, and arsenic segregation in germanium were used to study the influence of segregation and precipitation on the anomalous transmission. Working conditions of x‐ray diffraction microscopy are discussed, and it is shown that segregation phenomena produce typical impurity contrast which is reflection‐dependent if the impurities are in solid solution. For precipitated impurities, this relation does not exist.