Thin films of 10,12-pentacosadiynoic acid were prepared using Langmuir and spin-coating techniques and polymerized using a controlled dosage of UV radiation. The radiation-induced phase transitions: from the colorless monomer, via the metastable blue phase, to the red polydiacetylene phase, and finally to degradation of the material, were monitored by optical absorbance spectroscopy. Deconvolution analysis of the absorbance curves allowed us to monitor quantitatively the dynamical changes in the chromatic properties of the films as a function of applied UV radiation dose. Several reaction kinetics models were applied in order to describe the phase transitions in the films. The results present the phase evolution in PDA and compare the kinetics for Langmuir films vs. spin-coated films. Polymerization directly at the air-water interface was found to be two-to-three orders of magnitude faster compared to solid-supported films of the same material. Moreover, we show that the data of the solid supported films is considerably better fitted when a reversible intermediate phase between the blue and the red phases is considered. Furthermore, a shift of the Raman active triple bond supports the presence of the intermediate phase.