The interatomic array and bond structure in crystalline states of the monochalcogenides tin selenide and gallium selenide are described and correlated with their solid lubricating capacity. Friction tests assessing their solid lubricating performance were carried out on a pin-on-disk machine. Specifically, large crystalline pieces of each inorganic solid were cut and cleaved into flat squares and subsequently rubbed against sapphire balls. In another case, fine powders of gallium selenide and tin selenide luere manually fed into the sliding interfaces of 440C pins and 440C disks. For the specific test conditions explored, friction coefficients of the sapphire/ gallium selenide and sapphire/tin selenide pairs were approximately 0.23 and approximately 0.35, respectively. The friction coefficients of 440C pin/440C disk test pairs with gallium selenide and tin selenide powders were approximately 0.22 and approximately 0.38, respectively. For comparison, a number of parallel friction tests were also performed ...