Attention-deficit/hyperactivity disorder (ADHD) is one of the most common childhood mental health conditions. An ADHD diagnosis is associated with adverse near- and long-term outcomes such as academic difficulties, social deficits, and poor family relationships. Behavioral Parent Training (BPT) is a common evidence-based treatment for childhood ADHD associated with significant and large-magnitude improvements across multiple functional domains; however, nearly two-thirds of children exhibit persistent symptoms and impairment following treatment. Empirical evidence has established parents’ quality of BPT skill use as an important mechanism of BPT response, but less is known about the factors that predict parents’ skill use. The goal of the current study was to explore whether parental cognitive errors (i.e., distorted cognitions parents hold regarding their parenting and their child) and treatment engagement attitudes (i.e., how interesting and helpful parents find treatment) predict skill use quality. The current study comprised a secondary data analysis of a randomized control trial of the Child Life and Attention Skills (CLAS) program (N=199), a behavioral intervention for children with ADHD-Inattentive Presentation. First, an exploratory factor analysis was conducted to determine whether subdomains of cognitive errors exist within this population, and two factors were observed: parent behavior errors (i.e., distorted cognitions about parents’ own parenting behavior) and child behavior errors (i.e., distorted cognitions parents have regarding their child’s behavior). This two-factor structure was then utilized in mediation analyses to examine the effect of parent and child behavior errors on quality of skill use via treatment engagement attitudes. Results from these analyses demonstrated that parent behavior errors significantly predicted quality of skill use, such that parents with higher levels of parental behavior errors demonstrated lower quality skill use, but no evidence of mediation was observed. Finally, moderated mediation models explored the impact of parent psychopathology (i.e., parental ADHD and depression) on mediational relations and found no evidence of moderation. Findings suggest that targeting parental cognitive errors, particularly cognitions about parents’ own parenting behavior, may be a novel target to improve BPT skill use and child outcomes.