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Quantification of the relative composition of the isomers in a photochromic system at any irradiation time interval is a critical issue in determining absolute quantum yields. For this purpose, we have developed a simple and convenient protocol involving combination of UV-visible and infra-red absorption spectroscopy. Photochromic cyclization reaction of cis-l,2-dicyano-l,2-bis(2,4,5-trimethyl-3-thieny1)ethene (CMTE) is analyzed to demonstrate the efficiency of the proposed methodology. This approach is based on the fact that the two isomers show distinctive infra-red bands. Detailed investigations of the UV-visible and infra-red spectra of the mixture obtained at different irradiation times in CCl4supported by quantum chemical computations lead to the unambiguous estimation of molar absorption coefficients of the closed isomer (CF= 4650 L mol−1cm−1at 512 nm). It facilitates the first determination of absolute quantum yields of this reversible photochromic reaction in CCl4by fitting the UV-visible spectral data (ΦOF→CF= 0.41 ± 0.05 and ΦCF→OF= 0.12 ± 0.02 at 405 nm and 546 nm, respectively). [ABSTRACT FROM AUTHOR]