This thesis focuses on two parts of performance evaluation in an existing office building in Vienna. In the first part the energy expenditure caused by heating and cooling was analyzed in relation to the calculated energy demand included in the mandatory energy certificate from the building design phase. Therefore real data from the first two years in operation was compared with those ones from the certificate on a monthly bases. The second part of the work focuses on a detailed assessment of indoor comfort together with an exploration of user interactions. Therefore in a representative part of the building, thermal comfort and the office occupiers usage of the controlling options of the thermal room air conditioning system were measured and inquired between May and August of 2018. For the objective assessment of thermal comfort, measuring devices for air temperature, relative air humidity and CO2-concentration were installed in all offices of one floor in the building. In addition, contact sensors on doors and windows together with occupancy detectors were installed to keep track of the users behavior and interactions. Finally, the operative room temperature was specifically measured twice within the span of the study. Furthermore, a questionnaire-based evaluation of the users subjective thermal comfort within that unit, was conducted periodically. As a part of this, it was also assessed whether or not the office occupants made use of the possibility to override the automatic thermal room regulation and how important they believed it to be. Therefore, this thesis is able to draw a comparison between the subjective and objective evaluation of the thermal room condition, as well as document user ventilation habits and rate the offices air quality.