Optical fibers connected to a spectrophotometer are shown to be analytical devices for recording the kinetics of structural changes within soft materials deposited on the side surface of the fiber core. A portion of electromagnetic radiation propagating inside an optical fiber penetrates out of its core (evanescent field) and interacts with molecules located within the depth of penetration (∼λ/10). It can be absorbed, scattered or can excite molecules around the fiber core surface. To prove the concept, a cylindrical hydrogel film [poly-N-(isopropylacrylamide), PNIPA] was polymerized by an evanescent wave around the fiber core immerged into the hydrogel forming solution. The integral intensity of light passing through the fiber core covered with the dried PNIPA hydrogel film was recorded with time during its hydration to reveal tiny features of swelling kinetics. The volume transition temperature (∼32 °C) was determined for the PNIPA hydrogel by measuring integral intensity and spectral composition of light passing through the combined gel-on-fiber core system at different temperatures. This value agrees well with the one known from the literature. Above this temperature, the PNIPA hydrogel shrank absorbing/scattering the light in different spectral ranges with a temperature increase. The results are of great importance for practical applications such as the time-resolved spectroscopy of structural transformations within not only synthetic, but also natural soft materials (cells, organelles, microorganisms, etc.).