Current transport through ultrathin SrTiO3 (STO) barriers has been studied systematically with respect to its dependence on barrier thickness d=2–30 nm, temperature T, and voltage V in state-of-the-art planar YBa2Cu3O7-x/SrTiO3/Au (YBCO/STO/Au) heterojunctions with c-axis oriented YBCO layer. We identified different transport regimes: Elastic tunneling was observed for samples with a nominal barrier thickness of 2 nm, which represents our experimental minimum for obtaining insulating transport characteristics. Already for slightly thicker STO barriers, resonant tunneling and hopping via a small number of localized states begins to dominate the transport behavior. For d≫20 nm, a crossover to variable range hopping behavior is observed in the high-bias voltage regime as well as in the high temperature regime. A localization length of ∼0.46 nm indicating the spread of the localized states can be derived from these experiments. This value is close to the STO lattice constant and corresponds to a high density of localized states of nL∼6×1019(eV)-1 cm-3. In a free electron tunneling model, this corresponds to an average tunnel barrier height of ∼0.4 eV. © 2003 American Institute of Physics.