This study investigated the validity of a new observational procedure for assessing moth-er–father–adolescent conflict interactions. Addressing key gaps in the field, this procedure moves beyond mother–adolescent dyads and self-report questionnaires by offering a standardized, observational family-level approach. The procedure combines the Lau-sanne Trilogue Play – Conflict Discussion Task (LTP–CDT) with a coding system, the Family Conflict and Alliance Assessment Scales with Adolescents (FCAAS). In a sample of 82 two-parent families with adolescents aged 10 to 13, the FCAAS demonstrated excel-lent inter-rater reliability and good validity. Confirmatory factor analysis supported a the-oretically grounded two-factor structure. Criterion and construct validity were confirmed via self-reports of marital satisfaction and coparenting, respectively. Ecological validity was supported by family members’ self-reports. This tool offers promising research and clinical applications and may be extended to diverse populations to assess known-group validity.