Senior citizens' communities of practice (CoPs) in Taiwan focus on the sharing of experiences and expertise, and engagement with issues of mutual concern. Because most of their members have a wealth of social experiences, as well as the time and strong motivation to learn new skills, these CoPs have strong potential to become learning environments aimed at the sharpening of expertise. To better understand the senior citizens' interactions in such an environment, the research team conducted a qualitative study of six CoPs in Taiwan, guided by the following research question: What are the key factors to construct the learning environment of senior citizens' CoPs? The researchers visited selected CoPs, including two self-sponsored groups, two interest groups, and two volunteer groups. In total, 24 interviewees were recruited, and the semi-structured interview method was employed to explore how a learning environment is created for a senior citizen CoP. The findings highlight that a free-yet-practical communicative environment with a culture of respect for the individual fosters practice-oriented mutual engagement and joint enterprise. A facilitation-based method of building up senior-citizen learning environments is recommended. Collaborative tasks trigger comparisons and self-reflection. When sense-making activities work well, ample time will be needed to cement these activities into organizational routines, so that shared repertoires can be formed. The study also explained how the keys work as a whole.