Herein, we report the synthesis and structural characterization of K-8[(CO3)(3)Pu](2)(mu-eta(2)-eta(2)-O-2)(2)12H(2)O. This is the second Pu-containing addition to the previously studied alkali-metal peroxocarbonate series M-8[(CO3)(3)A](2)(mu-eta(2)-eta(2)-O-2)(2)xH(2)O (M = alkali metal; A = Ce or Pu; x = 8, 10, 12, or 18), for which only the M = Na analogue has been previously reported when A = Pu. The previously reported crystal structure for Na-8[(CO3)(3)Pu](2)(mu-eta(2)-eta(2)-O-2)(2)12H(2)O is not isomorphous with its known Ce analogue. However, a new synthetic route to these M-8[(CO3)(3)Pu](2)(mu-eta(2)-eta(2)-O-2)(2)12H(2)O complexes, described below, has produced crystals of Na-8[(CO3)(3)Ce](2)(mu-eta(2)-eta(2)-O-2)(2)12H(2)O that are isomorphous with the previously reported Pu analogue. Via this synthetic method, the M = Na, K, Rb, and Cs salts of M-8[(CO3)(3)Ce](2)(mu-eta(2)-eta(2)-O-2)(2)12H(2)O have also been synthesized for a systematic structural comparison with each other and the available Pu analogues using single-crystal X-ray diffraction, Raman spectroscopy, and density functional theory calculations. The Ce salts, in particular, demonstrate subtle differences in the peroxide bond lengths, which correlate with Raman shifts for the peroxide O-p-O-p stretch (O-p = O atoms of the peroxide bridges) with each of the cations studied: Na+ [1.492(3) A/847 cm(-1)], Rb+ [1.471(1) A/854 cm(-1)], Cs+ [1.474(1) A/859 cm(-1)], and K+ [1.468(6) A/870 cm(-1)]. The trends observed in the O-p-O-p bond distances appear to relate to supermolecular interactions between the neighboring cations.