1. A study was made of the behavior of several solid lubricants under dry sliding friction conditions at nominal loads of up to 60 kg/cm2 and temperatures of up to 400°C. It was established that, in ascending order, these rank as MgF2, ZnS·H2O, CaF2, and BaF2. 2. Assessed on the basis of the absolute value of its coefficient of friction and the character of the latter's variation as a function of load and temperature, zinc sulfide is markedly inferior as a solid lubricant to barium and calcium fluorides. 3. At 20, 100, and 200°C, the frictional properties of CaF2 and BaF2 are identical, but at 300 and 400°C BaF2 surpasses CaF2 in frictional performance.