A new pseudoternary phase, Cs5Sb8S18(HCO3), was prepared from supercritical ammonia at 160 degrees C over 4 days using stoichiometric quantities of Cs2CO3, Sb2S3, and S-8. Red parallelepipeds of the title phase crystallize in the centrosymmetric monoclinic space group C2/c, with a = 27.064(3) Angstrom, b = 11.634(4) Angstrom, c = 12.642(3) Angstrom, beta = 97.45(2)degrees, and Z = 4. The compound has a layered structure, with the layers formed from ribbons of condensed psi-SbS4 trigonal bipyramids linked via S-3(2-) chains, and both Cs+ and HCO3- ions between the layers, giving rise to the formula Cs5Sb8S12(S-3)(2)(HCO3). Each layer is approximately 10 Angstrom thick, with two layers per unit cell. The condensation of the group 15/16 building blocks into ribbons is the result of a combination of edge- and vertex-shared antimony(III) sulfide pseudo trigonal bipyramids. In this phase, the S-3 chains always link adjacent ribbons via apical coordination sites on the antimony atoms. The open cavities created by the linking trisulfide dianions between neighboring ribbons accommodate the antimony lone pairs and two unique cesium cations. A third cesium cation is located between layers. Unique to this compound, the anionic antimony sulfide layers are separated not only by cesium cations but also by bicarbonate anions. Results derived from infrared spectroscopy, diffuse reflectance, bond valence sums, and magnetic susceptibility measurements support the formulation of this compound as an electron-precise, wide-bandgap material. Thermal stability studies of Cs5Sb8S18(HCO3) are also discussed.