Size-controlled, catalytically active PVP-stabilised Pd nanoparticles have been studied by operando liquid phase XAS during the Suzuki cross-coupling of iodonanisole and phenylboronic acid in MeOH-toluene using KOMe base. XAS reveals nanoparticles are stable to metal leaching throughout the reaction, with surface density Pd defect sites directly implicated in the catalytic cycle. The efficacy of popular selective chemical and structural poisons for distinguishing heterogeneous and homogeneous contributions in Pd catalysed cross-couplings is also explored.