The effect of heating synthetic microcrystalline goethite at 60 ~ 80 ~ and 1050C was studied by X-ray powder diffraction, electron microscopy, weight-loss measurements, and M/Sssbauer spectroscopy. Heating led to no detectable changes in the unit-cell parameters or crystallite size (210, 150, and 170 in the (020), ( 110), and ( 120) directions, respectively), however, some of the X-ray diffraction lines were broadened due to an increase in microstrain in these crystallographic directions. The superferromagnetic transition temperature increased from 43 ~ to 46", 53 ~ and 54"C after heating to 60 ~ 80*, and 105"C, respectively, showing that the desorption of water from the surfaces led to an enhanced magnetic coupling among the crystallites.