Coadsorption of cesium and iodine on Pt(111) has been studied with Low Energy Electron Diffraction, Auger Electron Spectroscopy and Thermal Desorption Spectroscopy. By adsorbing I₂ onto a Cs layer with saturation coverage (with an incommensurate hexagonal close packed structure), two distinct structures were prepared. First, Pt(111)(4 × 4)–Cs,I was observed after dosing until the work function had increased by 1 eV. Further I₂ dosing gives mixed structures and then saturates at Pt(111)(7×7)R19.1∘–Cs,I. The Pt(111)(4× 4)–Cs,I was assigned to a Cs₂I 2D ionic crystal. Adsorption of Cs onto an I layer and the reactivity of selected coadsorption systems to oxygen were also investigated. Electrostatic calculations were performed to compare the stability of various structures, and it is shown that bilayer structures and mixed monolayer structures can both be consistent with ionic principles.