This study examined the adsorption of elemental S onclean Si(lOO) surfaces and Cs on s- covered Si(lOO) surfaces. The study was performed in an UHV system using LEED, AES and WF measurements. The objective was the protection of the\ surface against degradation. The s adsorption process may be enhanced by the addition of Cs. Results indicate that S adsortion forms: a hemisulfide, (~0.5 ML) with a (2xl) structure and a monosulfide, (~1 ML) with a (lxl) structure. Adsorption of Cs on clean Si(100)2xl reduces the WF to a minimum value with a subsequent increase towards the value of metallic Cs. Preadsorption of Son Si(100)2xl lowers the WF to a final plateau without the increase. The presence of S increases the binding energy and the maximum amount of Cs that can be deposited on the Si(lOO) surface. Structural models for S and Cs on Si(lOO) surfaces have been given in the text.