["Clinical Psychology &Psychotherapy, Volume 33, Issue 3, May/June 2026. ", "\nABSTRACT\nAlthough schema therapy (ST) is an effective intervention for individuals with personality disorders (PDs), its mechanisms of change are not yet well delineated. Understanding these mechanisms is essential for determining the key processes that optimize treatment efficacy. This study investigated mechanisms of change in day treatment group ST (DT‐GST) for patients with severe PDs. PD severity was the primary outcome in a multiple‐baseline single‐case design, assessed before and after DT‐GST, alongside idiosyncratic negative core beliefs measured weekly over 30 weeks. Secondary outcomes were early maladaptive schemas (EMS) and schema modes (SMs), measured pre‐ and post‐treatment. Mechanisms of change were examined through visual inspection, correlations and randomization tests. Changes in PD severity moderately correlated with changes in negative core beliefs (r = 0.57), but not with SMs. Negative core beliefs correlated positively with the Vulnerable Child (r = 0.76), Punitive Parent (r = 0.78), Demanding Parent (r = 0.61) and Detached Self‐Soother (r = 0.58) modes and negatively with the Happy Child (r = −0.77) and Healthy Adult (r = −0.57) modes. Meaningful change occurred across all treatment phases, with the greatest therapeutic progress in the second and third phases. This study highlights the central role of idiosyncratic negative core beliefs in reducing PD severity while also underscoring the importance of changes in SMs. Findings suggest that ST should prioritize reducing the Vulnerable Child mode and its associated EMS while reinforcing adaptive Healthy Adult and Happy Child modes and diminishing maladaptive Parent and detachment‐related coping modes. Further longitudinal research with repeated assessments is needed to clarify mechanisms of change in ST.\n"]