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.