During the research project "Quality of Sustainable Buildings" (English: QSB, German: QNB), 27 office buildings in Switzerland were investigated. Objective measurements (air quality CO2, air temperature, relative humidity) were performed and in addition, data on subjective judgements were collected using questionnaires. The analyses of this paper focus on the characteristics of ventilation (natural, mechanical and hybrid ventilation). Mechanically ventilated buildings indicated some advantages during the summer period with respect to the air temperature and in the winter with respect to the air quality (CO2). For naturally ventilated buildings, disadvantages during the winter were observed with respect to the air quality (CO2). The same buildings showed some advantage of a higher relative humidity in the winter period compared with the mechanically ventilated buildings. Subjective judgements did more or less correspond with the measured data. However, some parameters did not. Additional parameters for qualifying indoor air quality (here measured as CO2 concentration) need to be considered. In addition, the study showed a quite good quality (measured and judged) for most of the buildings. In order to contribute to energy savings and to achieve a better level of indoor air quality (e.g. humidity), indoor air temperature could be reduced in winter without negatively affecting the comfort requirements of the office users.