The substructure and the dislocation arrangement produced during polycrystalline solidification of 99.99% aluminum has been studied using the decanting method for the interruption of the solidification. Etching techniques to reveal individual dislocations and the method of thin epitaxial layers of aluminum oxide to reveal changes in impurity concentration have been employed simultaneously. The development of the structure towards the interior of the solid was studied by successive electrolytic polishings. In this way it was possible to show that the "cell substructure" formed by dislocations originates in areas where "microsegregation" of impurities has occurred. Towards the interior of the solid a dislocation rearrangement into a "macromosaic" substructure which is closely related to the cellular substructure occurs. At the same time a substructure of "macrosegregation" occurs, arising from the original grain boundaries. The results are interpreted in the light of recent theories of solidification.