Transorganizational systems (TSs) are defined as “coalition structures formed by two or more organizations for a common purpose” (Cummings, Blumenthal, & Greiner, 1983; p. 379). Although an extensive body of knowledge exists on TS outcomes, there has been little research exploring how TSs change over time. The thesis presents an exploratory, longitudinal, multi-method case study of a change initiative within a TS consisting of a police organization, a hospital, and a social service agency operating in a large city in Canada. The initiative involved the change to a process that is interdependently co-owned by all three organizations and relates to how these three partners deal with persons suspected of having mental illness in the community. Seventy-five semi-structured interviews (25 per organization) were conducted before and sixty semi-structured interviews (20 per organization) were conducted six months after the implementation of the planned TS change. The data from these interviews were supplemented with archival data from the Police. We draw from planned change (e.g. Burnes, 2004), TS (e.g. Crosby, Bryson, & Stone, 2006), and stakeholder (e.g. Mitchel, Agle, & Wood, 1997) literatures to analyze findings from this thesis. The results of this thesis indicate that the planned TS change has had a measurable impact on the TS. The results suggest that respondents’ perceptions of what they wanted to change (pre-change) and what they observed had changed (post-change) in the TS influenced how they made sense of the planned TS change, and that these sensemaking processes affected their views of interorganizational relationships. Our analysis also suggests that the planned TS change: (1) shifted power within the TS from the hospital to the police, (2) shifted urgency within the TS from the police to the hospital, and (3) eroded the legitimacy of the police in the eyes of healthcare workers.