Infrared diffuse reflectance spectra were recorded for the 22 tetrachlorodibenzodioxin isomers (TCDDs). By use of micro-DRIFT techniques and signal averaging, identifiable spectra for each of the isomers were achieved at low nanogram levels. Spectral features in the 1200/cm to 1600/cm region indicate that each isomer has a unique spectrum and is readily distinguishable from other isomeric TCDDs. Each TCDD isomer was correctly identified, from a user-created library of the 22 isomers, using a software algorithm. The uniqueness of the TCDD FT-IR spectra offers the previously unobtainable possibility of correct isomer identification, in the presence of isomeric chromatographic coelution interferences, by spectral subtraction. These DRIFT spectra were utilized to clarify ambiguous TCDD structural assignments. Utilization of these FT-IR techniques for environmental monitoring can complement the current procedure of gas chromatography/mass spectrometry (GC/MS) for dioxin characterization. (Copyright (c) 1985 American Chemical Society.)