Polymeric phases for carbon dioxide and carbon disulfide are predicted on the basis of density functional calculations. Poly-COO2/2 containing three-coordinated carbon atoms is calculated to be 22 kcal/mol less stable than free molecular CO2, while its sulfur analogue poly-CSS2/2 is 19 kcal/mol higher in energy than free CS2. Another polymeric carbon disulfide phase based on CS4/2 tetrahedra is calculated to be only 2 kcal/mol higher in energy than the trithiocarbonate-based polymer poly-CSS2/2. A three-dimensional CO2 silica-like compound may be derived from the corresponding CO4/2-based polymeric model which is higher in energy than free CO2 by 40 kcal/mol. Besides these solid-state compounds containing an extended network, four- and six-membered rings (CX2)(n) (n = 2, 3) have been calculated to be less stable than their corresponding monomers by less than 15 and 24 kcal/mol for X = S and O, respectively. The planar cyclic oligomers of CS2 should be feasible and could be the precursors in the synthesis reaction of the one-dimensional poly-CSS2/2 phase.