The use of UFAD (Underfloor Air Distribution) systems is increasing rapidly and reported advantages of UFAD include energy savings and improved indoor air quality. At present few publications documenting measurements of ventilation effectiveness and indoor environment quality in occupied buildings with UFAD have been identified. In this field comparison study we measured several aspects of the thermal environment in two buildings with different ventilation systems (under-floor air distribution and mixing) located in Montréal (Québec). The results are presented in terms of thermal stratification, predicted thermal comfort indices (VATD, limit to air speed and PMV/PPD), and IAQ index (CO₂ level and stratification). The aim of the study was to determine whether UFAD in practice results in improved ventilation effectiveness compared to typical overhead air distribution without affecting the thermal comfort. The study found that there was little stratification for UFAD under operating conditions and it performed as a mixing system, thus no improvement in ventilation effectiveness was identified in the occupied zone, in comparison to the reference mixing system. In addition the predicted thermal comfort in terms of VATD, air speed and PMV/PPD was similar to those obtained for mixing ventilation and was within the acceptable limits set by ASHRAE 55-2010.