The performance of single-molecule magnets is typically defined by two parameters: the effective barrier to slow magnetic relaxation, U eff, and the magnetic blocking temperature, T B. While these two variables are linked, only the latter variable, T B, directly reflects the performance of the single-molecule magnet in practical use. In contrast, U eff, the thermal barrier to slow magnetic relaxation, only correlates to T B when the molecule's magnetic relaxation behavior is perfectly Arrhenius in nature.