The present paper deals with the composition and oxidation stability of organomineral complex in aggregates of various sizes of red earth and alluvial soils with various fertility levels in Zhejiang Province. 1. For aggregates in diameter between 2-5 mm to 0.01 mm, the content of total organic matter increased with the increase in size of aggregates. The proportion of humus dissolved in 0.1 N NaOH and 0.1 M Na₄P₂O₇ mixture tended to decrease, and that of insoluble humin tended to increase with the increase in size of aggregates. The humic acid/fulvic acid increased with the increase in size of aggregates.2. For aggregates in diameter from 2-5 mm to 0.01 mm the proportion of G₁, fraclion (after Tyulin 1943) in total complex decreased, and the proportion of G₂ fraction increased with the increase in diameter of aggregates. The G₁/G₂ ratio decreased with the increase in diameter of aggregates. 3. The oxidation stability (Kos) of humus in larger aggregates tended to be lower than that in smaller aggregates in fertile soils. However, the oxidation stability (Kos) of humps in larger aggregates tended to be higher than that in smaller aggregates in infertile soils. It is suggested that the organo-mineral complex in larger aggregates of fertile soils consists of relatively fresh organic colloids, and that of infertile soils consists of the more stable organic colloids.