To obtain a polyester-based toner with both low-energy fusing performance and good storage stability, we investigated a toner with a core-shell structure comprising two different polyesters with a low glass-transition temperature (T-g) and a high T-g, respectively. In this study, to fabricate and control the properties of core-shell-structured toner, we used confocal Raman spectroscopy to study mutual diffusion at the interface between the polyesters used as the core and shell for their adhesion. The PG-PES and BPA-PES concentration profiles at the their interfaces indicated asymmetric behavior, which appeared to be due to the one-sided diffusion of PG-PES with a molecular weight less than 2000 g/mol. Measurements conducted using samples having a narrow molecular weight distribution showed that a decrease of the amount of the low molecular weight component was also effective in preventing one-sided diffusion. Furthermore, the value of the apparent mutual diffusion coefficient greatly changed in the range of several orders by changing the annealing temperature and the molecular weight. We demonstrated that the quality of toner could be improved by controlling the diffusion behavior at the interface in the core-shell structure. (C) 2017 Society of Plastics Engineers