The pentagonal K-10[(UO2)(5)(mu-O-2)(5)(C2O4)(5)] species have been identified as the building blocks of uranyl-peroxide nanocapsules. The computed complexation energies of different alkali cations (Li+, Na+, K+, Rb+, and Cs+) with [(UO2)(5)(mu-O-2)(5)(O-2)(5)](10-) and [(UO2)(20)(mu-O-2)(30)](20-) species suggest a strong cation templating effect. In the studied species, the largest complexation energy occurs for the experimentally used alkali cations (Na+ and K+).