The aim of this study was to investigate how irrelevant speech, temperature and ventilation rate together affect cognitive performance and environmental satisfaction in open-plan offices. In Condition A, neutral temperature (23.5 °C), low intelligibility of speech (high absorption and low masking sound level) and high fresh air supply rate (30 l/s per person) were applied. This was contrasted to Condition B with high room temperature (29.5 °C), highly intelligible speech (low absorption and high masking sound level) and a negligible fresh air supply rate (2 l/s per person). Sixty-five participants were tested. In Condition B, performance decrement was observed especially in working memory tasks. Based on subjective assessments, mental workload, cognitive fatigue and symptoms were higher and environmental satisfaction was lower in Condition B. It was concluded that special attention should be paid to the design of whole indoor environment in open-plan offices to increase subjective comfort and improve performance.