Cesium adsorption on Pt(111) and its coadsorption with iodine and oxygen is studied in this dissertation. The work function during Cs dosing decreases and at 3 eV ( θCs = 0.15) the surface undergoes surface transition between a disordered anomalous state (Pt(111)(anom)-Cs) and islands of a Pt(111)(2×2)-Cs causing a change in the slope of the work function curve. The work function curve reaches minimum at about 5.5 eV where the surface is fully covered with the Pt(111)(2×2)-Cs structure (θCs). Further Cs dosing results in a work function increase and the surface undergoes a phase transition to Pt(111)(√3×√3)-Cs. The Cs saturated structure (Pt(111)(ihcp)-Cs) has an hexagonal symmetry with the unit cell vector aligned with the [1,0] direction of the substrate. Cs in the anomalous state desorbs from the surface in a high-temperature TDS peak (> 1000 K). When the lock-in TDS detection technique is used, this peak appears to be phase shifted by 180° when compared to the desorption peak of normally adsorbed Cs (θCs > 0:15) . This phase shift is a consequence of a positive charge of desorbing Cs. The TDS and work function behavior were explained by a Monte Carlo desorption model incorporating different desorption behavior for all four observed adsorption phases. When O2 is dosed on a Pt(111)-Cs surface, the maximum coverage of oxygen bonded to the surface is significantly increased in comparison to Pt(111). Anomalously adsorbed Cs activates the O2 bond but does not interact strongly with coadsorbed O. However, when O2 is dosed on Pt(111)(ihcp)-Cs, the oxygen rst adsorbs to a sub-layer adsorption site and strongly interacts with Cs. The oxygen in this state is responsible for thermal stabilization of coadsorbed Cs. When iodine is coadsorbed on a Pt(111)-Cs surface, it also strongly interacts with and thermally stabilizes Cs. During the desorption of Cs,I layers, some Cs and I desorb together in the form of a CsxIy cluster. The surface structures observed by LEED during the coadsorption of Cs and I are in good agreement with atomic arrangements predicted for ionic layers. The validity of this conclusion and the general behavior of ionic layers was checked by an electrostatic energy calculation for various structures.