In recent decades numerous nonprofit organizations (NPOs) are undergoing the process of professionalization characterized by increased attention to efficiency, accountability, and the adoption of “business-like” tools and practices. At the same time, the majority of NPOs rely on volunteer labor. Most nonprofit scholars focus on traditional volunteering in which one volunteers doing something other than their professional work. These studies see volunteerism and professionalism as two completely separate systems. Meanwhile, the subset of skills-based volunteers, i.e., volunteers who provide professionally-related skills or services in nonprofit settings is understudied and their views of NPOs' professionalization are generally unknown. The present thesis aims to help fill this gap by exploring how skills-based volunteers perceive their roles and increasing the professionalization of nonprofit organizations. The study is guided by two research questions: (1) how do skills-based volunteers react to the rise of NPOs' professionalization? and (2) how do they navigate possible tensions between professionalism and volunteerism? The study is an ethnographic case study of skills-based volunteers at Engineers WithoutBorders Sweden. The data is collected through 14 semi-structured interviews with working and retired volunteers complemented by my participatory observations as a part-time employee at the organization. To analyze and interpret the study findings, the thesis employs a boundary work framework. The empirical findings indicate that, contrary to traditional volunteers, skills-based volunteers react to NPOs’ professionalization positively. The professionalization helps them to integrate their roles as volunteers and professionals, thus, facilitating boundary blurring. At the same time, working and retired skills-based volunteers perceive student volunteers as a barrier to professionalization, consequently, emphasizing differences in competencies and reliability between themself and students and performing demarcation work.