Purpose During the COVID pandemic, many hospitals had to mobilize reinforcement healthcare workers, especially in intensive care (ICUs). We investigated the perceptions and experiences of reinforcement workers deployed to ICUs, and the impact of deployment on their personal and professional lives. Methods For this qualitative study, a random sample of 30 reinforcement workers was drawn from 4 centres participating in the larger PsyCOVID-ICU study. Individual semi-structured interviews were held, recorded, transcribed and analyzed by thematic analysis. Results Thirty interviews were performed from April to May 2021 (22 nurses, 2 anesthesiology nurses, 6 nurses’ aides). Average age was 36.8±9.5 years; 7 participants had no ICU experience. Four major themes emerged, namely: (1) Difficulties with integration, especially for those with no ICU experience; (2) lack of training; (3) difficulties with management, notably a feeling of insufficient communication; (4) Mental distress relating to the unusual work and fear of contaminating their entourage. Conclusion Healthcare workers deployed as reinforcements to ICUs at the height of the pandemic had a unique experience of the crisis, and identified important gaps in organisation and preparation. They also suffered from a marked lack of training, given the stakes in the management of critically ill patients in the ICU.