Indoor air pollution is of a growing concern in China. The nation’s growing urban work force spends a majority of daily time in the office, thus office indoor air quality (IAQ) can be a key determinant of worker’s wellbeing. Yet, information on office IAQ in rapidly developing southwestern China remains scarce. To address this knowledge gap, we conducted an observational study based on continuous monitoring in six office buildings in Chengdu, Sichuan Province, to investigate the concentration and variability of indoor particulate matter (PM_2.5) and carbon dioxide (CO₂). Hourly indoor concentrations for PM_2.5 and CO₂ were 0-459 μg/m³ and 375-1102 ppm, respectively, with considerable intra-building and inter-building variability. Indoor PM_2.5 exhibited temporal association with ambient PM_2.5, while indoor CO₂ exhibits both diurnal and weekly patterns. Four out of the six buildings showed a reduction in indoor PM_2.5 during work hours, suggesting functional filtration systems. However, we observed daily accumulation of indoor CO₂, suggesting ineffective ventilation. Indoor PM_2.5 pollution could be of health concern as all buildings experienced days when indoor PM_2.5 concentration was above WHO recommendation for at least 50% of the day. Multivariate linear model predicts that every 1 μg/m³ increase in ambient PM_2.5 is associated with a 0.60 μg/m³ increase in indoor PM_2.5. Building-specific multivariate models show work time, weekday, outdoor PM_2.5 are significantly associated with indoor PM_2.5 concentration. We hope findings from this study can inform future indoor pollution mitigation strategies.