The United States is in the midst of an opioid crisis, with more than 130 people dying each day from an opioid overdose (Centers for Disease Control and Prevention [CDC], 2018a). Given the recent spike in overdose deaths associated with highly potent synthetic opioids (i.e., fentanyl), there are few signs of the crisis abating (CDC, 2018a). Compared to other age groups, the prevalence of both heroin and prescription opioid use is particularly elevated among young adults (Sharma, Bruner, Barnett, & Fishman, 2016). While there is a large body of evidence demonstrating the effectiveness of substance use treatment in general, only 40-60% of individuals remain abstinent in the year following treatment (McLellan, Lewis, O’Brien, & Klebler, 2000). Less attention has been given to the process of treatment, making it challenging to determine what works and what needs improvement. This information is necessary in developing a comprehensive understanding of the treatment process, which in turn, will inform treatment decisions and improve client outcomes. The current study adds to existing literature by assessing during-treatment change on four proximal outcomes measuring substance-related cognitions (i.e., abstinence self-efficacy and commitment to sobriety) and self-concept (i.e., internalized shame and self-compassion). It was hypothesized that participants would demonstrate significant changes in the favorable direction on all four proximal outcomes. A sample comprised of primarily young adults with opioid use disorders attending short-term residential substance use treatment were enrolled in a naturalistic longitudinal study and assessed at intake, mid-treatment, and discharge. Individual growth curve models were used to examine change over time at both the aggregate and individual levels on the four proximal outcomes. Several potential moderating influences (e.g., demographic information, substance use history, mental health symptoms) were explored to better understand initial status and responses to treatment. Results revealed significant changes in the expected direction on all four proximal outcomes. In addition, several variables emerged as significant predictors of initial status and rates of change. These results provide further evidence that residential substance use treatment is not only a viable option in reducing substance use, but also impacts other important outcomes relevant to clients’ functioning and overall well-being.