The transport properties at the Si-Al2O3 interface are probed directly on virgin silicon-on-sapphire (SOS) wafers. The measurement consists of the activation of the pseudo-MOS transistor, once the sapphire substrate has been aggressively thinned down to 30 mum. Typical transistor characteristics are measured and used to uncover the parameters that reflect the quality of SOS wafers: carrier mobility, density of interface traps, and residual doping.