Tri-2-ethylhexyltrimellitate (TOTM) is one of the commercially used plasticizers for poly(vinyl chloride) (PVC). In this work, the diffusion behavior of TOTM in PVC films is investigated through real-time ATR-IR spectroscopy at 25, 40, and 70 C, and the ratio changes of three C=O structures of TOTM are plotted as a function of diffusion time. TOTM itself is composed of two types of C=O structures: one CO structure represents a free state and the other represents an aggregated C=O state formed by dipolar dipolar interactions, as confirmed by the second derivative of FTIR and density functional theory (DFT). Furthermore, another new band related to C=O groups of TOTM was observed during diffusion experiments in PVC at different temperatures, which can be assigned to dipolar dipolar interactions between TOTM and PVC (interacting C=O structure). These results show that the diffusion coefficients of TOTM in PVC increase with temperature. Conversely, the equilibrium concentrations of TOTM in PVC films decrease significantly with temperature due to rapid molecular movements and less interacting C=O structures. Moreover, it is also found that free C=O structures more easily interact with C Cl groups of PVC at 25 degrees C, while aggregated C=O structures show prior contribution at 40 C, which is effectively confirmed by two-dimensional (2D) correlation infrared spectroscopy. At 70 degrees C, however, free and aggregated C=O structures can reach an equilibrium much more quickly, but the content of interacting C=O structures has to keep a lower level because of higher temperature.