The preparation of mixtures of the isomers of l-allyl-l, 2, 5-trimethyl-l-silacyclopentane and some of their reactions with electrophiles are described. Individual isomers and reaction products have not been isolated but have been characterized by'H and I3C NMR spectroscopy. Cleavage of the allyl group may proceed with retention, inversion, or loss of configuration at silicon according to the reagent employed.