AF demagnetization of thermoremanent magnetization (TRM) and saturation isothermal remanent magnetization ( IRM(s) ) are used to establish diagnostic criteria for primary TRM and for secondary magnetization in igneous rocks. In applying the approach to the interpretation of natural remanent magnetization (NRM), the demagnetization of IRM(s) documents the nature of the magnetic phases in the rock, while the demagnetization of the NRM gives information about the process of magnetization. The ratio of NRM: IRM (s) measures the efficiency of the magnetization process. ¿ American Geophysical Union 1988