Recent experimental studies have shown that the temperature necessary to demagnetize viscous remanent magnetization (VRM) in magnetite-bearing rocks is significantly higher than predicted by standard N¿el single-domain theory. The Walton time-temperature equation, which has been invoked by some to explain the enhanced thermal stability, actually relates intensities of VRMs acquired at different temperatures rather than describing the unblocking of a previously acquired VRM. We show that the low-field approximation (LFA) used in all previous formulations, which predicts equal VRM acquisition and decay rates, is not valid for a magnetite assemblage in the earth's field. A medium-field approximation (MFA) predicts significantly faster VRM acquisition, and therefore greater thermal stability in decay, for the larger particles in the assemblage. Conventional theory does apply to red-bed acquisition/decay experiments, since the LFA is valid for hematite. ¿ American Geophysical Union 1988