The 2017 Paediatric Research in Emergency Departments International Collaborative (PREDICT) Bronchiolitis Knowledge Translation (KT) Study, a cluster randomised trial in 26 Australasian hospitals, found targeted interventions provided over one bronchiolitis season effectively de-implemented five low-value practices (salbutamol, glucocorticoids, chest radiography, antibiotics and epinephrine) by 14.1% (adjusted risk difference, 95% CI 6.5% to 21.7%; p<0.001). A 2-year follow-up study found de-implementation was sustained. This process evaluation aimed to identify factors that influenced sustainability of de-implementation of these five low-value practices in PREDICT Bronchiolitis KT Study intervention hospitals and examine fidelity and/or adaptation of the targeted interventions over 4 years post intervention delivery (sustainment).Semistructured qualitative interviews were conducted, over 2021 and 2022, with a purposive sample of emergency department (ED) and paediatric inpatient clinicians. Data were analysed thematically into facilitators and barriers using the Consolidated Framework for Sustainability Constructs in Healthcare (CFSCH). The Framework for Reporting Adaptations and Modifications to Evidence-based Implementation Strategies was used to explore fidelity and adaptation.50 clinicians (nurses: n=26; doctors: n=24) from 12 intervention hospitals were interviewed. Eight themes were identified and mapped to three CFSCH domains: (1) organisational setting; (2) initiative design and delivery and (3) people involved. Facilitators were a culture of evidence-based practice, ongoing multimodal education, strong clinical leadership as unofficial champions and the previous effectiveness of the PREDICT Bronchiolitis KT Study interventions. Barriers were lack of paediatric trained ED staff, assumptions by senior clinicians that junior doctors can provide evidence-based bronchiolitis management, bronchiolitis not a current improvement priority and lack of bronchiolitis education sessions. Use of the targeted interventions reduced over time and, when used, was adapted locally.This study provides insights into factors influencing the sustainability of de-implementation of low-value care in acute care settings. Fostering an evidence-based practice culture, supported by senior leadership and ongoing multimodal education, supports sustainability of improvements in this setting.Australian and New Zealand Clinical Trials Registry No: ACTRN12621001287820.