Studies have shown that people feel more comfortable when they can control the environment in which they live and work. In a mixed-mode office building, this control is usually through openable windows, but window opening behaviour can have a significant impact on building energy performance. This monitoring study investigated window behaviours in a mixed-mode office building during the summer of 2016 in Southampton. Applying a mixed methods approach, 31 windows and 10 offices doors, movements were monitored using accelerometers. Concurrently indoor and outdoor environments were monitored and occupants' surveys undertaken. Results show a statistical relationship between window opening behaviour and indoor ambient and radiant temperature and CO2 levels. The reasons for opening a window temperature and humidity as reported from the occupant’s survey. Observed patterns of window opening behaviour did not match the building’s design strategy as users acted differently from what BMS advised. This will have a substantial impact on energy performance in summer.