X-ray absorption experiments on 3d transition-metal tracer ions in SrTiO₃ are presented. The absorption spectra of the tracer-ion changed upon reduction in the SrTiO₃. This change is due to an oxygen vacancy created at the tracer-ion site. This finding is supported by density-functional theory calculations, which prove that the oxygen vacancies preferentially are created at the tracer-ion sites. Using the chemical sensitivity of x-ray absorption spectroscopy, tracer ions can be used to detect oxygen vacancies in SrTiO₃ and possibly in other oxide systems. © 2009 The American Physical Society.