The indoor environmental quality within a classroom is linked to the health, comfort and performance of students. It is well established that there are classroom environments where indoor environmental quality is poor. Therefore, the rehabilitation of school buildings is assumed as an appropriate strategy. Consequently, some countries have sponsored nationwide programs for the rehabilitation of school buildings, whose result has been, in some cases, other than the expected. Classrooms performance in service conditions must be evaluated and, from the results, optimized solutions should be established and carefully designed and executed to have the desired effect. Thus, a research project was defined, aiming to assess the impact of retrofit on the indoor environmental quality of school buildings. The methodology used included the in situ measurement of temperature, relative humidity, carbon dioxide concentration and ventilation rates. A total of 24 classrooms of 9 school buildings (2 non-retrofitted and 7 recently retrofitted) were studied. A long term monitoring was defined with three measurement campaigns: winter, mid-season and summer conditions, each with three weeks length. The results confirmed that non-retrofitted schools need to improve their indoor environmental conditions and in retrofitted buildings mechanical ventilation systems are not being used, with important consequences on the indoor air quality. (C) 2014 Elsevier B.V. All rights reserved.