CSI is prepared by treating cyanogen chloride with sulfur trioxide, the product being distilled directly from the reaction mixture.
::SO  +  ClCN  →  ClSONCO
In this transformation, both the carbon and the nitrogen termini of CN are functionalized.
The structure of CSI is represented as ClS(O)-N=C=O.  It consists of two electron-withdrawing components, the chlorosulfonyl group (SOCl) and the isocyanate group (-N=C=O).  Because of its resulting electrophilicity, the use of CSI in chemical synthesis requires relatively inert solvents such as chlorocarbons, acetonitrile, and ethers.