Palladium(II) complexes with chelating N-heterocyclic ligands bearing uncoordinated nitrogen atoms are efficient catalysts in the hydroarylation of alkynes, giving selectively trans-hydroarylation of the triple bond. The catalytic efficiency of these systems is markedly dependent on the strength of the acid added as reaction promoter, thus suggesting the formation, via ligand protonation, of highly electrophilic metal intermediates, which are responsible for the observed selectivity.