INTRODUCTION: This study recognized the lack of information regarding recruitment and retention factors associated with implementing HIV vaccine trials from the perspective of participants. It aimed to describe the motives and experiences of 31 young adults who participated in a phase II HIV vaccine clinical trial conducted in Maputo, Mozambique. METHODS: This was an ancillary study with a mixed-method approach that employed a convergent design, combining both quantitative and qualitative methodologies. Data collection involved questionnaire surveys, in-depth interviews, and focus group discussions. Participants were assessed before and after learning whether they received the experimental vaccine or placebo. Thematic analysis was used for qualitative data, while descriptive analysis and statistical tests such as Fischer's test and McNemar's exact test were applied to quantitative data. The study also utilized the Health Belief Model to understand the decision-making process of participating in an HIV vaccine study. RESULTS: Most of our participants were young females, single, with limited financial resources. Participants joined the trial with the belief that they had a unique opportunity to help the fight against HIV and contribute to the research for the discovery of an HIV vaccine. Positive experiences related to trial participation include gaining knowledge about HIV and personal health and receiving risk reduction counseling. Participants reported blood collection as a negative experience and that they suffered social harm because of trial participation. Participants felt abandoned after the trial ended. CONCLUSION: Preventive HIV vaccine trials should integrate a social-behavioral component to assess reasons for participation and refusal in real-time. Providing ongoing personal attention is crucial for young individuals who have committed 1-2 years to trial participation, extending beyond the trial period. Implementing tailored strategies for HIV risk assessment and reduction during and after the trial is essential. Addressing these factors can enhance preventive HIV vaccine trial implementation.