Chemisorption trends established using LEED techniques have shown that, at least for tungsten and melybdenum, the most densely packed planes are not the most reactive. The body-centered cubic (110), (100), and (112) planes are the most densely packed in this system and they contain surface atoms in density ratios of 1.4:1.0:0.8. Adsorption studies on the molybdenum (112) plane were concerned mainly with O 2 , N 2 , H 2 , and CO. Oxygen was the most reactive gas on this particular surface and formed (2×1) , (2×1)+(1×2) , (1×2) , (1×3) , and C (4×2) patterns. The (2×1) , (2×1)+(1×2) , and (1×2) coverages were obtained upon exposing the sample at room temperature to oxygen while the latter two patterns were obtained by heating the sample after very long oxygen exposures. No adsorption was found upon room temperature exposure to nitrogen while room temperature hydrogen exposure gave rise to a (1×2) LEEDpattern.Carbon monoxide adsorbed only in an amorphous state at room temperature. Slight heating caused ordering of this amorphous structure resulting in complex LEEDpatterns. Changes in the intensity distribution of specularly reflected beams differed greatly depending on the gaseous species adsorbed.