The addition of different substituted allylsilanes 2 to unactivated alkynes 1 in the presence of catalytic amounts of HfCl4 or the EtAlCl2-TMSCl catalyst system produced in high yields the silylated 1,4-dienes 3 regio- and stereoselectively. The exclusive trans manner of addition was confirmed by analysis of crude reaction mixtures by H-1 NMR and capillary GLC methods. Good agreement of relative reactivities of reaction of various allylsilanes 2a-e toward phenylacetylene (la) in the presence of HfCl4 with the relative reaction rates of 2a-e with carbenium ions supported the involvement of cationic species 11 as a reaction intermediate. The mechanisms for the HfCl4 and EtAlCl2-TMS catalyzed trans-allylsilylation of alkynes are proposed.