Background: Participatory research has gained traction as an approach to unlock perspectives when creating scientific knowledge and to facilitate societal changes. By conducting research with people, participatory research strives to engage individuals' perspectives in designing, conducting, and disseminating the research. Nevertheless, few studies have unpacked how understandings of the studied phenomenon are shaped among diverse research partners and, concurrently, how different perspectives are combined. Nested within an overall participatory mixed methods study on aging with multiple sclerosis (MS), this qualitative study explores how understandings of aging with MS are shaped in encounters between university researchers, older adults with MS, and employees in a patient association. Methods: The study was collaboratively conducted in Denmark by three research partners: a group of older adults with MS, employees in a patient association, and university researchers. Data on how different understandings of aging with MS were represented and shaped during the three-year research process was generated through field notes, meeting minutes, focus group interviews, and individual interviews. The collected data was analyzed through a thematic network analysis. Results: The study demonstrates how different understandings of aging with MS were represented among the research partners when the research was initiated. These understandings were shaped prior to —and, therefore, outside—the research setting, drawing from the research participants' lived experiences, professional backgrounds, and organizational cultures or situated in larger societal narratives. Through a process centered on reflexivity among the engaged research partners, the understandings of what it means to age with MS was shaped and re-shaped and eventually merged into a more dynamic understanding of later life with MS where different perspectives could co-exist. Conclusion: The findings demonstrate that research partners, including older adults with MS and employees from a patient association, brought diverse understandings to the study. Reflexive practices enabled these perspectives to co-exist, enhancing engagement and transparency, and fostering a dynamic understanding of later life with MS. This highlights the value of reflexivity in evolving complex understandings within participatory research. Plain English summary: In recent years, participatory research has been increasingly utilized in various research fields (e.g., aging research) with the ambition of engaging a diverse group of research partners to leverage their perspectives and contributions. This approach aims to form a more nuanced understanding of the studied phenomenon and to identify insights useful for practice However, working in diverse research teams have also been found to be complex, and it is poorly understood how the perceptions of the different research partners shape the final research product. Based on a study conducted in collaboration with employees in a patient association, older adults with multiple sclerosis (MS), and university researchers, the present study aims to unfold how perspectives of aging with MS are represented and shaped doing a participatory research period. It occurred over a three-year period where data was collected through field notes, meeting minutes, focus group interviews, and individual interviews. The findings highlight how research partners represent different understandings of the studied phenomenon, which is embedded in their social, cultural, and professional background and which potentially influence their expectations of and contributions to the research process. Furthermore, the study demonstrates how engaging in a critical dialogue about expectations and understanding of the studied phenomenon can provide insight into which perceptions are represented, making the participatory research process more transparent. Lastly, the study conveys how such critical dialogue facilitates understandings and perspectives evolving during the research process into a potentially more dynamic understanding of the studied phenomenon. [ABSTRACT FROM AUTHOR]