Medium-sized hydrogenated silicon cluster ions, SinHx+ (10 < n < 50), have been produced using an ion trap filled with silane (SiH4) at a pressure of similar to 10(-6) Torr. Time-resolved time-of-flight mass spectroscopic measurements revealed that there are two kinds of fast, efficient, cluster growth occurring via repetitive increments of eight Si atoms. One cluster growth continued beyond Si54Hx+, which has a considerably larger n value than has been previously reported (n less than or equal to 10). Experimental evidence indicates that neutral SinHx molecules, where n = 6-8, formed from SiH4 and accumulated on the chamber wall, serve as the source material for both successive reactions.