The use of silicon on sapphire (SOS) as a substrate for YBa 2Cu3O7-δ allows the growth of thick (∼4000 Å) films without the thermally induced cracking characteristic of epitaxial films on bulk Si substrate. Epitaxy is sustained and reaction is prevented by an intermediate buffer layer of yttria-stabilized (YSZ). The transport critical current density is as high as 4.6×106 A/cm2 at 77 K, and surface resistance measurements at 4.2 K are reported. Microtwin propagation from Si into YSZ is shown not to occur.