Mentalization, that is the capacity to understand our and others' behaviors in terms of intentional mental states, represents one of the core features of personality disorders (PDs) and can be related to therapists' countertransference (CT) and interventions. Aims: The aim of the present work was to study the relationship between therapists' technique, therapists' CT, and patients' mentalization, in a sample of patients with PDs undergoing a 40-session program of sequential brief-adlerian psychodynamic psychotherapy, a psychodynamic therapeutic model specifically developed for the treatment of PDs in public mental health services. Method: Eighty-seven patients with PD and their therapists completed ratings of mentalization (mentalization imbalances scale and reflective functioning questionnaire), CT (therapist response questionnaire), and therapists' intervention (comparative psychotherapy process scale) at five different time points (Sessions 5, 10, 20, 30, and 40). Results: Results showed that patients' mentalizing problematics decreased over time. Moreover, we found a reduction of therapists' disengaged CT, and an increase in positive CT over time. Regarding therapists' techniques, psychodynamic-interpersonal interventions were on average higher than cognitive-behavioral, but both techniques were used increasingly over time. Our results also showed significant and clinically coherent interactions between therapist's CT and techniques and between patient's mentalization imbalance and therapist's response. Our results highlighted the importance of early stages in therapy, since the most significant relationships between the various process variables (patient's mentalizing imbalances, therapist's techniques, and emotional responses) are observed between t 1 and t 2 , corresponding to the initial phases of the treatments. Clinical implications will be discussed.