Reversible oxidation-reduction of cerium ions in a monoclinic Ce4Al2O9 aluminate was studied by XRD, TEM, temperature programmed reduction-oxidation and magnetic methods. It has been shown that nanocrystalline Ce4Al2O9 reacts with gaseous oxygen, undergoing at 20-300 degrees C transformation into a new, oxidized phase described in the same P2(1)/c space group, but with 10% smaller unit cell volume. Magnetic measurements revealed that the oxidized Ce4Al2O9 still contains similar to 20% of paramagnetic Ce3+ ions. The oxidized Ce aluminate may be reduced back to Ce4Al2O9 by heating in H-2 atmosphere to 1000 degrees C. The reversibility of the oxidation-reduction process of Ce4Al2O9 aluminate bears resemblance to the behaviour of fluorite type CeO2, and may be of interest for applications (e.g., in catalytic high-temperature oxidation reactions).