<p>The preparation of thin films is more difficult for nanometre-scale thickness films and requires methods such as preparing them from ultra-low concentration aqueous solutions. This study presents thin films of different thicknesses prepared at room temperature from ultra-low concentration aqueous solutions of Coumarin 334. The transmission spectra of these thin films were measured using the optical cavity technique, particularly in the context of broadband cavity-enhanced absorption spectroscopy (BBCEAS). The spectral analysis showed peak shifts in the spectra of nanometre-scale thin films formed from different concentrations. The Swanepoel method was used to determine the thickness of the Coumarin 334 thin films from these transmission spectra. Measuring the nanometre-scale thickness of the thin films prepared from ultra-low concentration aqueous solutions of nano-molar Coumarin 334 was challenging. This led to further spectral analysis of the effects of the concentration of Coumarin 334 on the thin film thickness, refractive index, and absorption coefficient. The decrease in thickness due to an increase in the concentration was associated with an increase in the absorption coefficients.</p>