Collateral work with parents is a widely adopted practice within child psychotherapy. Therapeutic process within these parent sessions has not been empirically studied or defined, despite a sizable process-outcome literature in both child and adult individual therapy. This link between research and practice is particularly important among manualized, child-focused treatments, where the proposed therapeutic action and clinical approach to parent work is defined according to distinct theoretical principles. To address this gap in the child treatment literature, the present study used the Psychotherapy Process Q Set to examine the in-session processes of parent sessions from 16 treatments of regulation-focused psychotherapy for children (RFP-C). RFP-C is a manualized, psychodynamic treatment for children with disruptive behaviors that consists of 16 child sessions and four collateral parent sessions. The parent-session process ratings were compared to existing adult therapy prototypes and the RFP-C child session prototype. Results indicated that observer-coded psychotherapy process in RFP-C parent sessions was most similar to a cognitive-behavioral therapy prototype and moderately correlated with both a supportive-expressive psychodynamic psychotherapy and a reflective functioning prototype. Observer-coded parent session process was distinct from the RFP-C child prototype. Limitations and directions for future research and clinical practice are discussed. The findings of this study indicate the need to intentionally examine process in parent sessions, both within RFP-C and across modalities, as these sessions have their own unique mechanisms of therapeutic action that ultimately may be additive with regard to child outcomes. (PsycInfo Database Record (c) 2021 APA, all rights reserved).