The dicationic acetylene platinum(II) complex [ P t ( P N P ) ( C 2 H 2 ) ] ( B F 4 ) 2 (PNP = 2,6-bis-(diphenylphosphinomethyl)pyridine) was generated in situ by ligand substitution from the ethylene complex [Pt(PNP)-(C 2 H 4 )](BF 4 ) 2 and was reacted with a series of arenes at low temperature. Only electron-rich arenes added across the coordinated C−C triple bond and gave the corresponding arylalkenyl complexes (E)-[Pt(PNP)(CHCHAr)]BF 4 (Ar = C 6 Me 5 , C 6 H 2 Me 3 -2,4,6, C 6 H 3 Me 2 -2,6, C 6 H 3 Me 2 -2,4). A slow E− Z isomerization of the arylalkenyl complexes was observed. Single-crystal X-ray structure analyses were obtained for both E and Z isomers of the pentamethylbenzene derivative. The E isomers of [Pt(PNP)(CHCHAr)]BF 4 (Ar = C 6 Me 5 , C 6 H 2 Me 3 -2,4,6) reacted with excess HBF 4 •Et 2 O to give the corresponding arylalkene complexes [Pt(PNP)(CH 2 CHAr)](BF 4 ) 2 , whereas the Z isomers did not undergo immediate protonolysis. Using (E)-[Pt(PNP)(CDCDC 6 Me 5 )]BF 4 it was shown that the stereochemistry of the C−C double bond in the protonolysis product depends on the nature of the acid anion HX (X − = Cl − , BF 4 − ). The catalytic hydroarylation was studied in solution by NMR spectroscopy. The reaction studies provide a more refined view of the individual steps proposed for the Friedel−Crafts type mechanism of the Pt II -catalyzed intermolecular hydroarylation of alkynes.