The addition of different substituted allylsilanes 2 tounactivated alkynes 1 in the presence ofcatalyticamounts of HfCl4 or the EtAlCl2−TMSClcatalyst system produced in high yields the silylated 1,4-dienes3 regio-and stereoselectively. The exclusive trans manner of addition wasconfirmed by analysis of crude reaction mixturesby 1H NMR and capillary GLC methods. Good agreement ofrelative reactivities of reaction of various allylsilanes2a−e toward phenylacetylene (1a) inthe presence of HfCl4 with the relative reaction rates of2a−e with carbeniumions supported the involvement of cationic species 11 as areaction intermediate. The mechanisms for the HfCl4andEtAlCl2−TMS catalyzed trans-allylsilylation ofalkynes are proposed.