Treatment of diphenyl-diethynylsilanes, (H 5 C 6 ) 2 Si(CϵC-RЈ) 2 (RЈ = -C 6 H 5 , -4-Me-C 6 H 4 ), with two equivalents of dialkylaluminium hydrides, R 2 Al-H (R = -CMe 3 , -CH 2 CMe 3 ), afforded the corresponding dialkenylsilanes (3-5) by hydroalumination. The mixed alkenyl-alkynyl compounds resulting from the reduction of only one CϵC triple bond occurred as intermediates. Two of these (1 and 2) were isolated and characterized by crystal structure determinations. They show close interactions of the α-carbon atoms of the ethynyl groups with the coordinatively unsaturated aluminium atoms and a cis arrangement of H and Al atoms across the C=C double bonds. cis/trans Rearrangement took place upon the [a]