Samples of YBa2Cu3O7−xwithxranging from ∼0 to 0.65 have been analyzed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in the midinfrared region (400–6000 cm−1). Spectral line shapes vary gradually as a function of oxygen stoichiometry, and the reflectance at 400 and 1000 cm−1decreases linearly with decreasing oxygen content. Spectra of samples that were incompletely synthesized or exposed to a 4% CO2atmosphere at 650 °C clearly indicated the presence of carbonate. DRIFTS is therefore a quick, nondestructive method for determining oxygen content in YBa2Cu3O7−xpowders, and for detecting carbonate species due to synthesis error or reaction with CO2‐bearing atmosphere.