Three well-characterized glass-ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations (pTRMs) in the temperature interval 400¿--350 ¿C for varying lengths of time (10 min to 5 days) and in magnetic fields from 0.5 to 5 Oe (0.05--0.5 mT). The thermal demagnetization behavior of the pTRMs changes with domain state in the sense that with decreasing coercivity (increasing grain size), demagnetization curves become less steep and may not completely demagnetize until heated to the Curie temperature. Numerical calculations based on N¿el's single domain (SD) theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures. We observe that experimental results do not match any of the theoretical predictions. There is no discernible field dependence of demagnetization curves for the SD and pseudo-single domain samples and even for the ''soft'' multidomain (MD) sample in the field range investigated. An important consequence for thermally derived paleointensity data is that they may not be entirely reliable, especially if the sample remanence is due to MD grains. ¿ American Geophysical Union 1988