The determination of the onset temperature for molecular reorganization in conjugated polymer thin films is highly relevant for understanding microstructural evolution and device optimization. In spite of the importance of this parameter, it is difficult to measure in a thin-film geometry as most techniques for characterizing thermal transitions require bulk samples. Here, we demonstrate the ability of UV-vis absorption spectroscopy to reveal not only the onset temperature of morphological changes but also the nature of morphological evolution, namely, whether the film is becoming more ordered or more disordered. Instead of monitoring the overall spectral variation of conjugated polymer thin films within the UV-vis range, the redistribution of optical oscillator strengths across different narrow wavelength ranges is evaluated. With fits based on Franck-Condon analysis, molecular-scale rearrangements as a function of annealing temperature can be visualized. To prove the generality of this method, various conjugated polymers, including homopolymers and donor-acceptor polymers, which have been employed in a range of devices, have been tested. The information obtained from our method has been compared with other thermal analysis methods such as differential scanning calorimetry to illustrate the superiority and sensitivity of our method toward previously undetected thermal events. The method is also used to study the thickness dependence of thin-film phase transitions, which is inaccessible by bulk-sensitive thermal analysis. The application of this new method is believed to facilitate further optimization of conjugated polymer-based functional devices by accurately quantifying thin-film thermal transition.y