In situ observations of the growth of the anhydrite (100) surface in contact with supersaturated aqueous solutions under conditions within the stability field of this mineral (60–120 °C) were conducted using a hydrothermal atomic force microscope (HAFM). Advancement rates were measured for  steps, the most stable ones on the anhydrite (100) surface. Isothermal data fit well to linear correlations between step advancement rate and supersaturation; the activation energy for step advancement is 73 ± 5 kJ/mol. This is not significantly higher than activation energies reported for the growth of gypsum (60–70 kJ/mol) and does not support that slow dehydration rates of aqueous calcium is responsible for the well-known difficulty to precipitate anhydrite crystals from supersaturated aqueous solutions at temperatures well above the anhydrite-gypsum equilibrium temperature. The role of structural factors that could inhibit the growth of anhydrite is discussed.