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Isocyanates are a class of organic compounds that contain one or more isocyanate functional groups, characterized by the formula R-N=C=O. This functional group is highly reactive due to the electronegativity difference between nitrogen and oxygen, and the cumulative double bonds. Isocyanates are primarily used as building blocks in the synthesis of polymers, most notably polyurethanes. These polymers find widespread applications in foams, coatings, adhesives, and elastomers.

The simplest isocyanate is hydrogen cyanide, HNCO, though it is highly unstable. More common are organic isocyanates, such as toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI). These are typically produced through the phosgenation of the corresponding amines. The reaction of an isocyanate with a compound containing active hydrogen atoms, such as alcohols or amines, forms urethane or urea linkages, respectively, which are the basis of polyurethane chemistry.

Exposure to isocyanates can pose health risks. Many isocyanates are respiratory sensitizers, meaning that repeated exposure can lead to the development of asthma-like symptoms. Skin contact can cause irritation and sensitization. Due to these hazards, strict industrial hygiene practices and personal protective equipment are essential when working with isocyanates.