Ten large, single crystals of hemoilmenite (yFeTiO3(1-y)Fe2O3) with y close to 0.54 were extracted from the self-reversed Pinatubo dacitic pumice erupted in 1991 and oriented with respect to the c axis of the hexagonal structure by means of an x-ray diffractometer. Hysteresis measurements show that c is the hardest magnetization axis while the softest axes lie in the basal plane. The hysteresis curves observed at room temperature in this plane suggest that in addition to the multidomain (MD) regions commonly seen under the microscope using the Bitter technique, there are also crystal regions which behave like single-domain (SD) or pseudosingle-domain (PSD) particles. As could be expected from the hysteresis measurements, the directions of natural remanent magnetization (NRM), saturation isothermal remanent magnetization (SIRM), and thermoremanent magnetization (TRM) all lie in the basal plane, regardless of the direction of the applied field. Thus the antiferromagnetically coupled spins of hemoilmenite of intermediate composition lie in the basal plane. The partial TRMs (pTRMs) are approximately reversed; that is, their direction is always more than 90¿ away from the applied field direction and most often close to 180¿. In fact, pTRM directions are discretely distributed along three directions in the basal plane which are 60¿ away from each other. This distribution indicates that the magnetic anisotropy is of magnetocrystalline origin. Both NRM and TRM are exceptionally resistant to alternating field (AF) with no decrease in remanence intensity up to 30 mT at least. This behavior, together with the fact that the Lowrie-Fuller test is SD-type, indicates that natural and laboratory TRMs are carried by non-interacting single-domain regions. We propose a self-reversal model based on negative exchange interactions in which the self-reversed TRM is carried by cation-ordered (ferrimagnetic) SD-like regions dispersed within a cation-disordered (antiferromagnetic with weak ferromagnetism) MD matrix carrying a weak, normal TRM. ¿ 2000 American Geophysical Union