Initial water adsorption and desorption isotherms (Type II) are reported for Au‐coated quartz‐crystal substrates at temperatures of 15.5°, 23°, 30°, and 39 ° using a quartz‐crystal oscillator with an ultimate sensitivity of ∠10−8 g/cm2. Hysteresis between adsorption and desorption is most probably related to Au porosity and thermal shock. Isotherms are shown to qualitatively fit the BET model for adsorption at low relative pressures (referenced to the condensation pressure P 0). From the computed BET parameters, the heat of adsorption is found to be ∠1 kcal/mole and the monolayer capacity at 23 °C is ∠0.2 μg/cm2.