Thiazolidine-2-carboxylic acid (β-thiaproline) is a proline analog. It is an important building block of β-lactam antibiotics. Its X-ray photoelectron spectra has been investigated. It has been reported as a physiological substrate of hog kidney D-amino acid oxidase. Thiazolidine-2-carboxylic acid can be synthesized from cysteamine and glyoxylate.