This study presents an exemplar for psychometric evaluation and modification of established measures when applied to new populations. Specifically, we describe the use of two subscales (Career Decision-Making Self-Efficacy Scale and Math/Science Self-Efficacy Scale) from the Middle School Self-Efficacy Scale (MSSES) as outcome measures in an intervention study of high school students. Several researchers have utilized the MSSES with high school students since it was developed by Fouad, Smith, and Enochs 20 years ago, but few studies have examined it for construct validity with a high school sample, even though the measure was designed for a middle school population. Our findings demonstrated that the MSSES required modification for high school students in order to meet the standards of reliability and validity in a counseling intervention study. The discussion focuses on implications for career counseling and research, limitations of the findings, and suggestions for future research.