Plane-wave density functional theory and extended Huckel calculations have been used to study the adsorption of acetylene and hydrogen on the (111) surface planes of Pd, Pt, Ni, and Rh. In agreement with previous experimental and computational studies, atomic hydrogen is found to preferentially adsorb in a 3-fold hollow site on each metal surface, although the potential-energy surface for hydrogen binding is apparently rather flat. Differences in the adsorption behavior of acetylene on the four surfaces are more pronounced. An adsorption structure in which C2H2 is oriented above a 3-fold hollow, with its axis parallel to but tilted away from a metal-metal bond, is computed to be most stable on Pd, Pt, and Rh. On Ni(111), acetylene is found to strongly adsorb above two contiguous hollow sites, with its molecular plane perpendicular to the surface and bisecting a Ni-Ni bond. The anomalous adsorption behavior on Ni is explained using the crystal orbital Hamilton population formalism. Calculated C2H2 adsorption behavior is compared with experimental studies of acetylene decomposition and hydrogenation on single-crystal surfaces. Implications for the mechanism of acetylene hydrogenation on supported metal catalysts are also discussed.