This secondary-analysis register report aims at testing the role of emotion in the intervention effect of an experimental intervention study in academic settings. Previous analyses of the National Study of the Learning Mindset (Yeager et al., 2019) showed that in a randomized controlled trial, high school students who were given the growth mindset intervention had, on average higher GPA than did students in the control condition. Previous analyses also showed that school achievement levels moderated the intervention effect. This study further explores whether the emotion students experienced during the growth mindset intervention plays a role in the intervention effect. Specifically, using a sentence-level automated text analysis for emotional valence (i.e. sentiment analysis), students’ written reflections during the intervention are analyzed. Linear mixed models are conducted to test if valence reflected in the written texts predicted higher intervention effect (i.e. higher post-intervention GPA given pre-intervention GPA). The moderating role of school achievement levels was also examined. A 10% random sample of the data was analyzed as a pilot study for this registered report to test for feasibility and proof-of-concept. Results of the pilot data showed small, yet significant relations between emotional valence and intervention effects. The results of this study have implications on the role of emotion in the results of intervention or experimental studies, especially those that are conducted in academic settings. This study also introduces a user-friendly text-based analytic method for experimental psychologists to detect and analyze sentence-level emotional valence in an intervention or experimental study.