Phthalaldehyde (sometimes also o-phthalaldehyde or ortho-phthalaldehyde, OPA) is the chemical compound with the formula C6H4(CHO)2.

This pale yellow solid is a building block in the synthesis of heterocyclic compounds and a reagent in the analysis of amino acids.

[4] A more modern synthesis is similar: the hydrolysis of the related tetrabromo-o-xylene using potassium oxalate, followed by purification by steam distillation.

[5] OPA is used in a very sensitive fluorogenic reagent for assaying amines or sulfhydryls in solution,[6] notably contained in proteins, peptides, and amino acids, by capillary electrophoresis and chromatography.

OPA reacts specifically with primary amines above their isoelectric point Pi in presence of thiols.