Oxygen radial (O-2-, O-)-accommodated 12CaO(.)7Al(2)O(3) (C12A7) releases oxygen molecules at about 800 degrees C when heated in helium. The corresponding weight loss nearly accords with that calculated from an amount of the accommodated radicals, providing solid evidence that they are formed by an oxygen gas uptake from the atmosphere. Small difference between the observed and calculated losses suggests the presence of peroxide ion, O-2(2-), which is confirmed by the enhancement of 770 cm(-1) band in Raman spectra of the radical-accommodated samples.