The aim of this study was to evaluate the indoor (I) and outdoor (O) levels of NO₂, speciated volatile organic compounds (VOCs) and carbonyls at fourteen primary schools in Lisbon (Portugal) during spring, autumn and winter. Three of these schools were also selected to be monitored for comfort parameters, such as temperature and relative humidity, carbon dioxide (CO₂), carbon monoxide (CO), total VOCs, and both bacterial and fungal colony-forming units per cubic metre. The concentration of CO₂ and bioaerosols greatly exceeded the acceptable maximum values of 1800 mg m⁻³ and 500 CFU m⁻³, respectively, in all seasons. Most of the assessed VOCs and carbonyls occurred at I/O ratios above unity in all seasons, thus showing the importance of indoor sources and building conditions in indoor air quality. However, it has been observed that higher indoor VOC concentrations occurred more often in the colder months, while carbonyl concentrations were higher in the warm months. In general, the I/O NO₂ ratios ranged between 0.35 and 1, never exceeding the unity. Some actions are suggested to improve the indoor air quality in Lisbon primary schools.