The molecular ionic crystal [Cr3O(OOCCH=CH2)(6)(H2O)(3)](3)[alpha-PW12O40]center dot 15H(2)O [Ia] with stable three-dimensional micropores and a minimum aperture of 3.3 angstrom was synthesized with a phosphododecatungstate [alpha-PW12O40](3-) (polyoxometalate, POM) and a macrocation with acrylate ligands [Cr3O(OOCCH=CH2)(6)(H2O)(3)](+). The porous structure of la was basically constructed by an arrangement of macrocations forming a six-membered ring: vinyl groups (CH=CH2) of adjacent macrocations were aligned parallel to each other, suggesting a weak dispersion force between them. A guest-free phase [Cr3O(OOCCH=CH2)(6)(H2O)(3)](3)[alpha-PW12O40] [Ib] was formed by the treatment of Ia in vacuo at room temperature without any structure change. Compound Ib showed shape-selective sorption of CO2 and C2H2 (molecular size = 3.3 angstrom) over N-2 (3.6 angstrom) and methane (3.7 angstrom), and the sorption enthalpy of C2H2 was larger than that of CO2. The high affinity toward C2H2 was further confirmed as follows: the Monte Carlo simulations of the optimized geometries of C2H2 in Ib showed that both hydrogen atoms were in the vicinity of the surface oxygen atoms of POMs. The gas sorption profiles showed a much faster diffusion for C2H2. All these results suggest that the anion and cation mainly play the guest-binding and structure-directing roles, respectively, (i.e., concerted functions) in an ionic crystal with stable three-dimensional micropores.