A variety of measurements indicates that Au nanoparticle- catalyzed Sonogashira coupling of iodobenzene and phenylacetylene is predominantly a heterogeneous process. Large gold particles are much more selective than small ones, which is consistent with this view. Substantial leaching of Au into the solution phase occurs during the reaction, but the resulting supernatant liquid exhibits immeasurably low catalytic activity; TONs for the nanoparticles are orders of magnitude higher than those for the leached Au, once more pointing to the primacy of heterogeneous chemistry. These properties are independent of the support material, implying that they are intrinsic to metallic Au nanoparticles. Reaction data and quantitative analysis of the solid and solution phases by XPS and ICP-MS, respectively, showed that catalytic activity ceased when all the metallic Au had dissolved. Conversely, when starting with a soluble Au complex, a long inactive induction phase is followed by the sharp onset of reaction and steadily increasing catalytic activity, consistent with the eventual nucleation and growth of gold particles. Again, the implication is that, for the nanoparticle-catalyzed reaction, heterogeneous catalysis is by far the most important process.