Etch pits 50 and more molecular layers deep have been observed on the surface of natural molybdenite crystals exposed to oxygen at temperatures in the region of 650°C. The geometry and manner of growth of the etch pits suggest that etching takes place through the intermediary of a two-dimensional gas of adsorbed oxygen molecules on the surface of the crystal. For the five samples studied this explanation of the etch pits leads to the conclusion that the fraction of adsorption sites occupied by adsorbate molecules lies in the range θ=2.3×10−3−3.6×10−2. The average activation energy of a molecular etch reaction is 33.7 kcal/mole or 1.43 eV.