The purpose of this study is to provide a framework for spectroscopic investigation of an enhancement mechanism of the interfacial adhesion using a polyvinyl chloride (PVC) matrix as a model. PVC dissolved in a tetrahydrofuran solution was dropped onto carbon fiber (CF) or glass fiber (GF), and the interfacial shear strength was measured after annealing at various temperatures. The interfacial shear strength of PVC/CF increased significantly with the annealing temperature compared with that of PVC/GF. As characterized by Raman spectroscopy, the polyene content (an indicator of PVC degradation) in bulk PVC was positively correlated with the interfacial shear strength. PVC radicals were not detected at the annealing temperature in the electro spin resonance spectroscopy. It was demonstrated that the interaction between the ions (C+Cl−) formed during the decomposition of PVC (formation of polyene) and the π electrons in the benzene of CF; the cation–π interaction is attributable to the enhancement of the interfacial shear strength.