UV-induced graft polymerization is a simple and effective method for the surface modification and functionalization of polymeric materials. The structure of poly(acrylic acid) (PAAc) grafted onto a polymeric surface by UV-induced graft polymerization of acrylic acid was investigated using optical waveguide spectroscopy (OWS). The PAAc graft layer exhibited a large increase in thickness in water compared to that in the dry state due to the swellability of the PAAc chains. Its thickness further increased with increasing the pH of the aqueous solution. This could be attributed to the increased electrostatic repulsion of the carboxylate anions on the PAAc chains, as the ionization degree of the carboxylic groups increases with pH. The thickness of the PAAc graft layer is dependent on the graft condition employed, including the UV intensity and irradiation time, and the AAc monomer concentration. On the basis of these data, we propose a model for the structure of the PAAc graft layer and a PAAc layer modified by galactose ligands.