Goethite maximum blocking temperatures (60--150 ¿C) are governed by the amount of isomorphous subsitution in the goethite lattice. The goethite dehydration reaction (250--350 ¿C) is attended with a concurrent remanence decrease in finely intergrown hematite due to a change of the original hematite structure promoted by the liberated water vapor. This remanence decrease leads to a bending point in the thermal decay curve, which could be easily mistaken for the presence of another magnetic consituent. The magnetic properties of the minerals formed through goethite dehydration (hematite and traces magnetite) strongly depend on their growth history. ¿ American Geophysical Union 1988