The H 2/Pt(111) system has been studied with LEED, ELS, thermal desorption spectroscopy and contact potential measurements. At 150 K H 2 was found to adsorb with an initial sticking coefficient of about 0.1, yielding an atomic H:Pt ratio of about 0.8:1 at saturation. H 2/D 2 exchange experiments gave evidence that adsorption is completely dissociative. No exrea LEED spots due to adsorbed hydrogen were observed, but the adsorbate was found to strongly damp the secondary Bragg maxima in the I/ V spectrum of the specular beam. The primary Bragg maxima were slightly increased in intensity and shifted to somewhat lower energy. A new characteristic electron energy loss at -15.4 eV was recorded upon hydrogen adsorption. The thermal desorption spectra were characterized by a high temperature (β 2-) state desorbing with second order kinetics below 400 K and a low temperature (β 2-) state that fills up, in the main, after the first peak saturates. The β 2-state is associated with an activation energy for desorption E∗ of 9.5 kcal/mole. The decrease E∗ with increasing coverage and the formation of the β 1-state are interpreted in terms of a lateral interaction model. The anomalous structure in the thermal desorption spectra is attributed to domains of non-equilibrium configuration. The work function change Δϑ was found to have a small positive maximum (∼ 2 mV) at very low hydrogen doses (attributed to structural imperfections) and then to decrease continuously to a value of -230 mV at saturation. The variation of Δϑ with coverage is stronger than linear. The isosteric heats of adsorption as derived from adsorption isotherms recorded via Δϑ compared well with the results of the analysis of the thermal desorption spectra.