The adsorption of oxygen on a clean Ni60Fe40(100) alloy surface yields a c(2×2)O LEED pattern near half-monolayer coverage; this is accompanied by Fe segregation. The intensity of this pattern decreases (reversibly) near 525 °C with no change in oxygen coverage; this is due to an order–disorder transition in the O overlayer. Sulfur was deposited using a beam of S2 molecules from an electrochemical cell. S-dosed, annealed samples showed negligible alloy segregation and exhibited p(2×2)S or c(2×2)S LEED patterns with varying degrees of surface order, depending on sulfur coverage. Upon exposure to O2 gas, a p(2×2)S pattern was gradually replaced by a c(2×2). The brightest c(2×2) pattern occurred at a total S+O coverage of 0.5 monolayers. LEED studies of the c(2×2) patterns for several S–O compositions at fixed half-monolayer coverage suggest that metastable O-rich and S-rich c(2×2) domains are formed during coadsorption and become partially miscible with temperature cycling.