Reaction of A-alpha-PW(9)O(34)9- with YCl3 in an aqueous Na2CO3 solution produces a dianion-encapsulated A-type sandwich polyoxometalate, (YOH2)(3)(CO3)(A-alpha-PW9O34)(2)(11-). The X-ray structure of this complex reveals that three Y(III) ions are sandwiched between two A-alpha-PW9O349- moieties and that a carbonate dianion is encapsulated in the same plane as the three Y(III) atoms. The oxygen atoms of the CO32- are sitting at the midpoints of the sides of the triangle formed by the three Y(III) ions. P-31 and C-13 NMR studies confirm that this complex is significantly more stable than the analogous A-type sandwich polyoxometalates containing divalent metals.