O/O exchange and subsequent time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis was employed to investigate the transport of oxygen, and thus the behavior of oxygen vacancies, in [nominally undoped, (100) oriented] single-crystal SrTiO substrates. Isotope exchange anneals were performed as a function of temperature, 948 /K 1123, at an oxygen activity O 0.50 and as a function of oxygen activity, 0.01 O 0.70, at 1073 K. All isotope profiles show the same characteristic form: an initial drop over tens of nanometers close to the surface, which is attributed to an equilibrium space-charge layer depleted of oxygen vacancies, followed by a profile extending several microns into the solid, which is attributed to diffusion in a homogeneous bulk phase. The entire isotope profile can be described quantitatively by a numerical solution to the diffusion equation with a position-dependent diffusion coefficient; the description yields the tracer diffusion coefficient in the bulk (∞), the surface exchange coefficient , and the space-charge potential . All (∞) data are consistent with nominally undoped SrTiO substrates being weakly acceptor doped; the activation enthalpy for the migration of oxygen vacancies in bulk SrTiO is found to be ≈ 0.6 eV. The surface termination of the SrTiO substrates was seen to affect significantly the surface exchange coefficient . Values of obtained as a function of and O are approximately 0.5 V, indicating strong depletion of oxygen vacancies within the equilibrium surface space-charge layers. Thermodynamic modeling indicates that space-charge formation at the TiO-terminated (100) surface is driven by the Gibbs formation energy of oxygen vacancies at the interface being lower than in the bulk.