The chemical composition studies of indoor particulate matter (PM_2.5 and PM_0.1) are lacking in the developing world, yet the attention to indoor air pollution and occupant health risks is rising. This study therefore aims to investigate the chemical compositions of indoor and outdoor PM_2.5 and PM_0.1 in residential and school buildings in Vietnam during different seasons and the effects of emission sources on particle characteristics. The results show that the concentration of indoor PM_2.5 in the residential house was higher than that outdoors, and the indoor-to-outdoor ratios (I/O) exceeded 1, indicating the contribution of indoor sources. The peak concentration of indoor particles was observed during the incense burning days, whereas a higher indoor organic carbon concentration may be due to cooking activities. A similar variation of indoor and outdoor PM_2.5 indicates the penetration of outdoor particles. In the school building, seasonal variations in indoor PM_0.1 and its chemical species were observed, suggesting outdoor sources had a significant effect. Several factors may affect I/O and the infiltration factor of PM_0.1, such as indoor sources, air exchange rate, cracks in the door and window, and particle deposition rate. The reason for the I/O value slightly above 1 for PM_0.1 in this study remained unclear. Further investigations are recommended to obtain knowledge regarding the penetration of particles, particularly PM_0.1. The findings provide a better understanding of the chemical components of indoor particles and the effects of emission sources, which is crucial to developing management measures for indoor air quality and mitigating exposures.