This report involves the room and elevated temperature ultrahigh-vacuum study of Se adsorbed on caesiated Si(111)-7×7surfaces using Auger electron spectroscopy, low-energyelectron diffraction, work-function measurementsand thermal desorption spectroscopy. Adsorption of Se onCs/Si(111)-7×7 surfaces initially occurs on the uncaesiatedportions of the Si substrate and subsequently on the Cs adatoms.Heating the Se/Cs/Si(111)-7×7 surface causes Se to diffuse belowthe Cs adlayer to form strong dipoles with the alkali metaladatoms. The presence of Se increases the energy of binding of Csto the substrate, and CsxSeySiz compoundformation was observed with a binding energy of 2.8 eV/atom.The Cs-Se bonds most probably break at 1050 K, and two bindingstates of Cs and Se were observed after heating to temperatures >1050 K. Their calculated binding energies are 3.0 and3.2 eV/atom. The coadsorption of Cs and Se induces a highdegree of surface disorder, while desorption most probably causessurface etching. Finally, the presence of Cs on the Si(111)-7×7surface greatly suppresses the formation of SiSe2, detectedwhen Se is adsorbed on clean Si(111)-7×7 surfaces.