This thesis presents a field assessment of thermal stratification, ventilation effectiveness and thermal comfort in an office with an underfloor air distribution system. In an earlier study, thermal stratification, ventilation effectiveness and thermal comfort were evaluated with this arrangement, but diffusers were later relocated away from occupants and furniture. The opportunity was taken to investigate and compare the performance of the system before and after relocating the diffusers. Moreover, the underfloor air distribution system investigated in the study building had a unique configuration of ducted horizontal discharge floor diffusers that has been rarely documented in the literature. Even though underfloor air distribution systems with horizontal discharge diffusers are expected to improve ventilation effectiveness compared to those with swirl diffusers and mixing ventilation systems, this study found no difference. The underfloor air distribution system investigated in this research did however create stronger stratification than has been reported in literature for underfloor air distribution systems with swirl diffusers. Thermal comfort conditions in the study building were found to comply with ASHRAE standard 55. This suggested that the underfloor air distribution system with horizontal discharge diffusers could maintain a thermally comfortable environment while increasing the potential for energy savings compared to underfloor air distribution systems with swirl discharge diffusers, due to stronger stratification profiles, and without compromising the ventilation effectiveness seen with other systems.