Hydrosilylation, hydrostannation, carbosilylation, and carbostannation of unactivated alkynes with organosilanes, or organostannanes proceed effectively in the presence of catalytic amounts of Lewis acids to produce the corresponding vinylsilyl or vinylstannyl compounds in a highly regio- and stereoselective way. Although it is well known that transition metal catalyzed hydrometalations and carbometalations, in general, proceed in a cis-manner, the Lewis acid-catalyzed reactions proceed in a trans-manner exclusively. The coordination of a triple bond to Lewis acids is proposed as a key step for the Lewis acid catalyzed reactions.