A single crystal of zeolite K-12.5-A was prepared by exposing K-A at 350 degrees C to 0.3 Torr of potassium vapor, and its structure was determined by X-ray diffraction methods in the cubic space group Pm3m, At 350 degrees C, K-3(2+) clusters form in at least 50% of the sodalite cavities. No excess potassium atoms (and therefore no clusters) are found in the large cavities. The triangular K-3(2+) cluster interacts primarily with framework oxygens, but the effect of the one delocalized electron can be seen in the longer K-O bend lengths (2.78(2) as compared to 2.59(2) Angstrom in dehydrated K-A) and shorter K-K distances (4.35(6) as compared to 4.70(3) Angstrom in K-4(3+)). Due to considerations of partial occupancy, the K-K distance in K-3(2+) is likely to be shorter than 4.35 Angstrom (perhaps as little as 4.0 Angstrom), and its approach to framework oxygens is Likely to be longer than 2.78 a (perhaps as long as 2.9 Angstrom).