A aeries of alkyl (phenyl) iso-butenylsilanes have been prepared by the reaction of iso-butenyl-magnesium bromide with different alkyl (phenyl) chlorosilanes in tetrahydrofuran and their physical constants were determined. With the same .method, we have prepared iso-butenyl triethogysilane. The latter was easily hydrolyzed and polymerized. When iso-butenylmagnesiurn bromide and tetraethozysilane were mixed in proportion of 2:1, no diiso-butenyl diethoxy-ailane was obtained.