A series of well-defined homopolymer-stabilized Pd colloids with varying metal particle size was used to study the Heck coupling reaction between aryl halides and olefins. Correlation of initial reaction rates and particle sizes determined by transmission electron microscopy demonstrates that the Heck reaction is a sensitive probe of metal surface structure. The presence of the colloid-stabilizing homopolymer affords a much more stable catalyst than that obtained using the colloid-precursor metal complexes alone. Extremely high total turnover numbers (TON, moles of substrate/moles of Pd; TON = 100 000) and turnover frequencies (TOF = TON/h; TOF > 80 000) are easily attainable with poly(vinylpyrrolidone)-stabilized colloidal palladium in the coupling of p-bromobenzaldehyde with butyl acrylate.