Reactions catalyzed by palladium(II) acetate and trifluoroacetic acid (TFA) have a clear preactivation phase. However, the structure of real catalytic species remains unclear. We show that the key species are cyclic trinuclear complexes of composition [Pd₃(OAc)_6−x(OTFA)_x] (x = 1–6) formed by a sequential ligand exchange from [Pd₃(OAc)₆]. Furthermore, we prove that the trinuclear palladium backbone of the precatalyst remains preserved during the first phase of the C–H activation reaction of acetanilides. In other words, the reaction pathway including the trinuclear species should be taken into account in discussion about mechanisms of the reactions catalyzed by palladium acetates.