Natural ventilation has the potential to provide cooling and fresh air and cut 40% of the total energy consumption of European office buildings. While in the milder seasons natural ventilation is an obvious low-energy choice, if poorly designed it can cause overheating in summer and poor air quality in winter. In order to promote the use and design of naturally ventilated (NV) buildings, it is therefore important to understand how current NV buildings perform in terms of thermal comfort and indoor air quality. Only through long-term studies is it possible to evaluate seasonal variations in environmental performances and understand how they affect both occupant perception and adaptation. This is especially important in NV office buildings where high internal heat gains and occupant high density make it more difficult to ensure an optimal performance throughout the different seasons. However, longitudinal field studies are under-represented in the current literature. In this study we look at the performance of eight offices in four naturally ventilated buildings in the temperate oceanic climate of UK. Both objective (measurements) and subjective (surveys) data is collected through a long-term monitoring campaign of thermal comfort and indoor air quality. The results of the first six months of monitoring are reported in this paper. Air quality is found to be highly correlated to the outdoor weather conditions, with indoor air quality worsening in the winter months, especially in the office with the highest number of occupants and where a proper control strategy is not implemented. In terms of thermal comfort, middle floor offices are found to be the best performing spaces. The results of this preliminary analysis suggest that high density NV offices need to have a natural ventilation control strategy in place in order to ensure an optimal performance in terms of air quality. Concerns over overheating problems in roof-exposed offices are also highlighted.