Passivation of InAs surfaces in InAs/GaSb type-II strained layer superlattices is important in many applications, and cysteamine has been effectively used for this purpose. However, there is a lack of fundamental understanding of the bonding between cysteamine and InAs. Here, first principles modeling, X-ray photoelectron spectroscopy (XPS), and high temperature XPS were used to characterize the adsorption of cysteamine on InAs surfaces. Our surface modeling results on InAs(1 0 0) showed that the computed adsorption energies were highest for In-S and As-S bonds (both approaching 43 kcal/mol), and these values were influenced by the local reconstruction of the InAs(1 0 0) surface and, in some cases, the presence of neighboring cysteamine molecules. XPS results of as-received, control and cysteamine treated InAs(1 0 0) surfaces were consistent with In-S and As-S bonding of cysteamine and indicated an indium-rich surface. High temperature XPS results up to 250 degrees C showed that sulfur persisted at the interface at higher temperatures while carbon and nitrogen were removed, suggesting that the In-S and As-S bonds are more stable than the C-S bond. Finally, near edge X-ray absorption fine structure (NEXAFS) measurements confirmed that the primary amine group of cysteamine was available for covalent bonding with potential potting materials.