Pathway of diethyl phthalate photolysis in sea-water determined by gas chromatography-mass spectrometry and compound-specific isotope analysis

► The major intermediates were mono-ethyl phthalate and phthalic anhydride. ► 13C-enrichment (Δδ13C = 12.3‰) in residual DEP was a direct evidence of photolysis. ► Results of CSIA indicated the initial reaction step was cleavage of a CO bond. ► The degradation pathway was proposed for DEP photolysis.