XPS, TEM, and reaction studies were used to examine the catalytic behavior of gold species deposited on lanthana toward the cross-coupling of phenylacetylene and iodobenzene. Atomically dispersed Au(I) and Au(III) were catalytically inert, whereas metallic Au(0) nanoparticles were both active and very selective. Thus it is metallic gold and not ionic gold that provides the catalytically active sites. Au(0) nanoparticles supported on silica, gamma-alumina, and BaO were active but relatively unselective; however, as with lanthana, ceria-supported Au(0) nanoparticles showed high selectivity. This strong promoting effect of the lanthanide oxide supports on Sonogashira selectivity cannot be accounted for in terms of acid/base, redox, or SMSI effects; it may be tentatively ascribed to metal --> support hydrogen spillover.