The heterobifunctional, photoreactive crossslinker benzophenone-4-isothiocyanate enables one to covalently link two target groups. The isothiocyanate chemically reacts with amines and upon UV illumination, the photoreactive component reacts with nucleophiles or forms C-H insertion products. Crosslinking reagents are useful for probing the spatial relationships and interactions within and between biomolecules.