A fundamental problem in rock magnetism and paleomagnetism is the physical origin of anomalously elevated unblocking temperatures. To investigate this problem, we have studied viscous remanent magnetization (VRM) acquired at 225 ¿C by 1--2 ¿m magnetite particles synthesized with the glass-ceramic method. Experiments were performed with two primary goals: (1) to determine which particular conditions produce anomalously elevated unblocking temperatures, and (2) to compare various observed aspects of viscous behavior to predictions based on two different physical mechanisms that might raise the unblocking temperature: the reduction of magnetic relaxation time by the applied field (the Neel medium-field mechanism), and directional ordering of defects within domains and domain walls. When compared to model predictions, the present results indicate that neither the Neel medium-field mechanism nor the directional ordering mechanism satisfactorily accounts for anomalously high unblocking temperatures. Surprisingly, however, the unblocking temperature spectrum of VRM is critically sensitive to a sample's initial state. When samples first are thermally demagnetized from 580 ¿C to 225 ¿C, VRMs acquired subsequently at 225 ¿C exhibit pronounced high-temperature ''tails'' extending to 450 ¿C. Yet when samples first are demagnetized in an alternating field at room temperature before acquisition at 225 ¿C, their VRMs unblock by approximately 300 ¿C. Despite these two very different thermal responses, VRM acquisition curves at 225 ¿C are virtually identical for the two initial states. Furthermore, zero-field storage prior to acquisition suppresses VRM acquisition coefficients by similar amounts for both initial states. Significantly, standard thermal demagnetization to room temperature greatly attenuates the high-temperature tail of a VRM acquired subsequently at 225 ¿C. We conclude that thermal response of domain structure, as determined by thermomagnetic history, plays a critical role in the thermal unblocking process. ¿ American Geophysical Union 1993