The adsorption of hydrogen on Cs-modified Al(111) surfaces has been studied using high-resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). A stabilized hydrogen species which desorbs simultaneously with Cs at 480 K (gamma hydrogen) appears in the presence of Cs and the ratio between the number of Cs and that of the gamma hydrogen is 1:2, independent of Cs coverage, which is indicative of a substantial attractive interaction between Cs and the gamma hydrogen. HREEL spectra of the gamma hydrogen exhibit intense loss peaks at around 800 and 1700 cm(-1). Detailed analyses of these loss peaks using isotope mixtures (H+D) indicate that the gamma hydrogen is directly bound to Al with a geminal structure (AIH(2)). From the combined results of TPD and HREELS, it is considered that a cesium aluminum dihydride complex (CsAIH(2)) is formed on the surface, which is supported by ab initio calculations.