Impurity heterogeneity has been studied along a nearly compensated silicon crystal. The doping impurities were phosphorus and aluminium. The validity of quantitative measurements on the whole sample (such as Hall effect, mass spectrometry, nuclear activation) have been checked by qualitative methods of microobservation (X‐ray microprobe, electron microscopy). A theoretical model has been set up which tentatively explains why impurities may precipitate in different ways.