We report on capacitance-voltage (C-V) analysis of SrTiO3/GdTiO3 heterostructures that contain a high-density, two-dimensional electron gas (2DEG) at the interface. The complex admittance was measured as a function of frequency for different gate biases applied to a Schottky contact on the SrTiO3. A one-dimensional, complex impedance transmission line model was used to extract C-V characteristics from the frequency dependent admittance. The extracted capacitance was nearly independent of the gate voltage (up to −0.5 V), indicating a fully depleted SrTiO3 layer. The results are used to estimate the dielectric constant of the SrTiO3, the degree of modulation of the 2DEG by the maximum applied voltage (∼2.5%), and to establish an upper limit of the residual carrier density in the bulk of the SrTiO3 film (∼9 × 1018 cm−3).