Soil formation usually increases magnetic susceptibility, most often by increasing the concentrations of magnetite and maghemite, which are two ferrimagnetic iron oxides. Here we provide evidence that magnetic enhancement in aerobic soils not affected by detrital magnetic inputs or thermal transformation of other iron oxides is mostly due to the formation of maghemite, which is later transformed into hematite-the iron oxide that gives red color to soil. We show that the maghemite/hematite ratio is influenced by the particular environment and the degree of soil development, so it constitutes an effective tool for paleoenvironmental and planetary studies.