Electron-energy-loss measurements in 2-nm-sized regions of single-crystal YBa2Cu3O7 show about 0.5 wt. % carbon dissolved in the matrix. The energy-loss near-edge fine structure (ELNES) at the carbon 1s absorption edge favors an environment similar to BaCO3. The carbon ELNES is correlated with the oxygen 1s ELNES in a way that suggests that carbon-derived orbitals participate in the electronic structure within 1 eV of the Fermi level. © 1991 The American Physical Society.