The optical properties of 7-hydroxy-4-methyl coumarin immobilized on polyethylene glycol support and dissolved in distilled water solution are compared with the optical properties of the dye dissolved in ethanol and discussed on the basis of the formation of different chemical species of the dye molecule. The presence of the polymeric support affects both the absorption and emission features of the coumarin by changing the chemical equilibrium among the different species typically observed at the ground and excited states. The optical properties of synthesized PEG-supported coumarins are able to warrant biomedical applications in water-based environment and enable us to gain insight into the photophysical activity of coumarins; in particular, the steric hindrance of the polymeric support inhibits, as expected, the formation of the tautomeric form of the compound in the excited state, a further proof of the single-step neutral to tautomer reaction in coumarins.