The synthesis and characterization of dislocation-free, undoped, single crystals of Si enriched in all three stable isotopes is reported: Si-28 (99.92%), Si-29 (91.37%), and Si-30 (89.8%). A silane-based process compatible with the relatively small amounts of isotopically enriched precursors that are practically available was used. The silane is decomposed to silicon on a graphite starter rod heated to 700-750° C in a recirculating flow reactor. A typical run produces 35 g of polycrystalline Si at a growth rate of 5 μ m/min and the silane to Si conversion efficiency exceeds 95%. Single crystals are grown by the float-zone method and characterized by electrical and optical measurements. Concentrations of shallow dopants (P and B) are as low as mid-10(13) cm(-3). Concentrations of C and O can be lower than 10(16) and 10(15) cm(-3), respectively. © 2005 The Electrochemical Society. All rights reserved.