Accurate thermodynamic parameters of thin films of materials are crucial in understanding their behavior in the nanometer scale. A new and simple method for determining the vapor pressure and thermodynamic properties of nanometer thick films of materials was developed based on UV-absorbance spectroscopy. Well-characterized benzoic acid was used to calibrate the spectrometer and the experimental conditions. The thermodynamic properties of pentaerythritol tetranitrate (PETN) were determined to validate the use of this new method. The estimated values of the thermodynamic parameters of PETN are in excellent agreement with the values reported using the most widely used Knudsen effusion method for determining vapor pressure lower than 1 pascal. The elegance of this method is its simplicity. The results indicate that UV-absorbance spectroscopy is a model-free and powerful technique in determining thermodynamic parameters in the nanoscale.