Spectral change of a cationic dye 3,3’-diethylthiacyanine iodide (THIA) in the presence of an anionic surfactant AOT has been presented. The THIA-AOT system exhibits blue-shifted metachromasia at AOT concentrations below its critical micellar concentration (CMC) and is thought to be due to the aggregation of electrostatically bound dye-AOT complex (DS). Metachromasia is gradually reversed to the monomeric band (peak at 425 nm) by AOT above its CMC. Principal component analysis (PCA) method has been applied for spectral analysis; the results show that the metachromatic peaks at 377 and 366 nm originate, respectively, from trimer and hexamer of the dye associated with AOT. From PCA, the molar absorption coefficient spectra of the individual absorbing components and the equilibrium constants for the systems, monomer reversible arrow trimer and hexamer reversible arrow monomer below and above CMC of AOT, respectively, have also been obtained. The micellar aggregation number of AOT obtained from PCA is found to be 16 which is in good agreement with the literature value.