In the field, the "terra-roxa" soil presents a structure of small granules, of stable aggregates. The massapé soil shows a subangular structure, medium to large, of stable aggregates. The analytical results of the laboratory confirm these descriptions of the field, showing that the condition of aggregation of these soils, as defined according to the distribution of the aggregates by size classes, presents remarkable differences: the massapé soil contains a greater amount of rough aggregates than the "terra roxa", and its stability in the water is also higher. The physico-chemical characteristics (texture, moisture, pH, organic matter, N, Ca, Mg, K and exchange capacity) determined in all sizes of aggregates, showed that those of "terra-roxa" present a clear and constant variation of the clay content in function of the size: the larger and the smaller aggregates present the higher contents, and the medium ones the lower contents. All the other analysed values show variation to the same effect. The aggregates of "massapé" soils have got practically the same clay contents in all size classes, as well as values similar to the other analysed elements. Of the analysed elements, only the oxyds free of iron did not show this behavior. There was also stated a rather distinct ratio between the contents of clay and the values of organic matter, sum of bases and exchange capacity, in all classes of size of the studied aggregates. The results obtained show that the aggregates of "terra roxa" and "massapé" soils have well defined physico-chemical characteristics which distinguish them remarkably. The authors try to study the influence of several elements analysed in the formation and stability in the water of the aggregates.