Detailed micromagnetic, rockmagnetic and mineralogical investigations on dacitic pumice from the eruption of Mt. Pinatubo (1991, Philippines) showing reversed NRM/TRM are presented. Two intergrown hemoilmenite-phases in chemically zoned particles were detected as being responsible: a rim-(or covering) phase consisting of a weak-ferromagnetic (disordered) hemoilmenite phase (low saturation magnetization but magnetically hard like hematite, FeTiO3 content 53--57 mol percent) and a core-phase consisting of a ferrimagnetic (ordered) hemoilmenite phase (high saturation magnetization but magnetically softer constant FeTiO3 content ≈58 mol percent). Preliminary studies were carried out in order to elucidate the magnetic interactions (possibly exchange coupling) responsible for the self-reversal of the NRM/TRM. A preliminary and schematic model is presented which summarizes the main points of acquisition of a self-reversed TRM according to our results. ¿ American Geophysical Union 1996