Objective: Optimizing a self-persuasion intervention app for adolescent HPV vaccination requires investigating its hypothesized mechanisms. Guided by the experimental medicine approach, we tested whether (a) self-persuasion intervention components (verbalize vaccination reasons, choose HPV topics) changed putative mechanisms (memory, autonomous motivation) and (b) measures of the putative mechanisms were associated with HPV vaccination. Method: These are secondary analyses from a randomized 2 (cognitive processing: verbalize reasons vs. listen) × 2 (choice: choose HPV topics vs. assigned) factorial trial (Tiro et al., 2016). Undecided parents (N = 161) with an unvaccinated child (11-17 years old) used the self-persuasion app, recalled reasons for vaccination (memory measure), and completed an autonomous motivation measure. Adolescent vaccination status was extracted from electronic medical records 12 months postintervention. Results: The verbalize component resulted in greater recall accuracy of vaccination reasons (p < .001); however, the choose topics component did not increase autonomous motivation scores (p = .74). For associations with HPV vaccination, recall accuracy was not associated (ps > .51), but autonomous motivation scores significantly predicted vaccination (ps < .03), except when controlling for baseline motivation (p = .22). Conclusion: The intervention app engages parents in reasons for vaccination; however, memory may not be a viable mechanism of vaccination. Although the intervention did not affect autonomous motivation, associations with vaccination status suggest it is a viable intervention target for HPV vaccination but alternative strategies to change it are needed. Future testing of a refined app should examine implementation strategies to optimize delivery in clinical or community settings. (PsycInfo Database Record (c) 2022 APA, all rights reserved).