More than scratching the surface: The mechanism by which supported Pd nanoparticles (PdNPs) catalyze cross-coupling reactions is the subject of debate. The changes in supported PdNPs during coupling reactions are studied by exploiting modified carbon nanotubes (CNTs) as support materials. After catalysis, CNTs with scratched walls and PdNPs with surface crystalline distortions were discovered, offering insights into the catalytic mechanism.