An X-ray absorption fine structure (XAFS) study has been performed to examine the chemical form of Ca in spinach. The spectra obtained by XAFS measurements show direct evidence that Ca exists in the chemical form of calcium oxalate in spinach, which is an insoluble salt. This result can explain the low bioavailability of Ca in spinach. The bioavailability is discussed from the viewpoint of the solubility of calcium oxalate in the stomach.