Thiol (also called SH, mercaptan, or sulfhydryl group) is a reactive group that is useful for bioconjugation. Thiols are highly nucleophilic. When ionized in alkaline conditions, they are very reactive against Michael acceptors (like acrylates), alkyl iodides, and other electrophiles. One of the most selective and efficient reactions is Michael's addition of thiols to maleimides.

Another useful property of thiols is their ability to react with metallic gold. While gold is highly inert, this reaction occurs spontaneously with the evolution of hydrogen. This can be used to cover gold nanoparticles and obtain self-assembled monolayers (SAMs) on gold wafers.