Employees in the service sector deal with a variety of emotional job demands due to interactions with clients. Emotional job demands often result in heightened levels of emotional exhaustion and decreased levels of performance. The current study aims to explore whether the adaptive behavioural pattern of psychological flexibility diminishes the negative impacts of emotional job demands on emotional exhaustion and subsequent performance. Data were collected from 116 nonprofit service workers, using self-report questionnaires (i.e., baseline) and diaries (i.e., follow-up). The results suggest that psychological flexibility is negatively associated with emotional exhaustion and positively associated with performance. In addition, psychological flexibility is found to attenuate the negative effects of emotional job demands on emotional exhaustion and performance. Finally, the results reveal that the attenuating role of psychological flexibility diminishes if employees are already exhausted. The results support the importance of personal resources, like psychological flexibility, in buffering the negative effects of emotional job demands on emotional exhaustion and performance. However, employees no longer benefit from high levels of psychological flexibility when facing high levels of exhaustion due to excessive emotional job demands.