The use of low‐energy (10–250 ev) electron diffraction as a method of studying the chemisorption of gases on metal single crystals is described. The apparatus permits controlling the temperature of the crystal in the range from liquid nitrogen to room temperatures during observation. Outgassing the crystal is effected by strong heating using electron bombardment. The residual gas structures on the (100) faces of copper and nickel single crystals are determined to be: (1) a monolayer with a single‐spaced square array, found on both copper and nickel after heating at temperatures slightly below the minimum temperature for observable evaporation; and (2) a double‐spaced, face‐centered square array, found after heating at or above this temperature. Nitrogen adsorbs on the (100) face of a copper single crystal at 25°C and −78°C into an incomplete single‐spaced square array. Most of the adsorption at 25°C occurs in a pressure range of 10−4to 10−1mm Hg in a ten‐minute interval, with little change occurring between 10−1and 8 mm Hg. At −78°C, the corresponding pressure range is 10−4to 10−3mm Hg with little adsorption occurring between 10−3and 1 mm. At room temperature, oxygen adsorbs into a complete, single‐spaced, square array at a pressure of 2×10−3mm Hg in a ten‐minute interval. At higher pressures, additional oxygen adsorbs into a thicker layer of no regular structure.