Thermal demagnetization of viscous remanence (VRM) and partial thermoremanence (pTRM) of 20- and 135-&mgr;m natural magnetites reveals a broad spectrum f(TUB) of unblocking temperatures TUB, both > and <TB, the blocking temperature. In contrast, 0.04-&mgr;m single-domain (SD) grains demagnetize sharply at TUB=TB. High- and low-TUB tails of f(TUB) were wider for 135-&mgr;m multidomain (MD) grains than for 20-&mgr;m PSD grains. 10-mT alternating-field demagnetization rendered the VRM of the 20-&mgr;m magnetite more SD-like in subsequent thermal cleaning, selectively erasing both low- and high-TUB tails of f(TUB). Primary and secondary remanences are then cleanly separated, and non-linear Thellier paleointensity determination becomes linear. Our results agree well with (TB,TUB) data for 105-year thermoviscous overprints in the Milton Monzonite over the range of log(time) common to the two studies. Because (TUB)av≈TB for PSD and MD as well as SD grains, the Pullaiah et al. paleothermometry method will work if (TUB)av values are used instead of (TUB)max values. ¿ 2000 American Geophysical Union