A simple model is developed to explain the often-observed diminished values of the magnetic hyperfine fields in microcrystals. It is shown that below the superparamagnetic blocking temperature thermally excited oscillations of the magnetization around an energy minimum reduce the average magnetization and the magnetic hyperfine splitting in the Mössbauer spectrum. In a microcrystal of volume V these quantities are reduced by a factor of about 1- kT/(2κ V), where kT is the thermal energy and κ is related to the anisotropy constant. Mössbauer spectra of 60 Å, 100 Å, and 120 Å Fe3O4 particles and 120 Å α-Fe2O3 particles show excellent agreement with the theory.