Platinum is relatively reactive, compared to silver and copper, which prompted us to study the growth and structure of a thin insulating layer on Pt(111). We grew monolayer islands of NaCl and studied them with scanning tunneling microscopy (STM) and atomic force microscopy (AFM). STM images of the islands revealed a square lattice which we confirmed, via density functional theory (DFT) calculations, to be the Cl anions, similar to other surfaces. Surprisingly, however, the AFM images appeared to only resolve approximately two-thirds of the Cl ions. DFT calculations showed that the adsorption heights of the Cl anions above the surface have a bimodal distribution in which they are either approximately 60 pm above the Na ionic plane or 40 pm lower. An electrostatic model shows that this structure reproduces the AFM observations.