Single-crystal X-ray diffraction has yielded new crystal structures for cesium formate (CsOOCH) and rubidium formate (RbOOCH). The cesium formate structure has the same unit cell and space group as that published from powder X-ray diffraction data ( Masuda, Y.; Yahata, A.; Ogawa, H. Inorg. Chem. 1995, 34, 3130 - 3133) but differs radically in the placement and orientation of the formate ions. The new crystal structure has been successfully modeled with an empirical force field based on pair potentials, whereas it proved impossible to develop a force field that gave an adequate description of the powder structure. For rubidium formate, the gross structure is similar to that previously published ( Masuda, Y.; Morita, W.; Yahata, A.; Yukawa, Y. Thermochim. Acta 1998, 318, 39 43), but the space group includes a mirror plane (Pbcm rather than Pbc2(1)). From this information, we have been able to analyze the effect of the cation size on the crystal structure for alkali-metal formates.