Ortho-phthalaldehyde (OPA) is a dialdehyde in which two formyl groups are attached to adjacent carbon atoms of a benzene ring. It is used as a high-level disinfectant and sterilant for medical and dental equipment, including endoscope reprocessing, where it acts as a cross-linking agent for proteins. It is also used as a reagent that forms fluorescent conjugation products with primary amines for the detection of amines, peptides and proteins. It is a pale yellow crystalline powder or clear liquid. Use in biocidal products depends on the applicable product authorisation.