This paper presents a Social Identity Model of Organizational Change (SIMOC) and tests this in the context of employees' responses to a corporate takeover. This model suggests that employees will identify with the newly emerging organization and adjust to organizational change more successfully the more they are able to maintain their pre-existing social identity (an identity maintenance pathway) or to change understanding of their social identity in ways that are perceived as constituting identity gain (an identity gain pathway). We examine this model in the context of an acquisition in the pharmaceutical industry where 225 employees were surveyed before the implementation of the organizational change and then again 18 months later. In line with SIMOC, pre-change identification predicted post-change identification and a variety of beneficial adjustment outcomes for employees (including job satisfaction, organizational citizenship behavior, lower depression, satisfaction with life, and post-traumatic growth) to the extent that either (a) they experienced a sense of identity continuity or (b) their supervisors engaged in identity leadership that helped to build a sense that they were gaining a new positive identity. Results showed a negative impact of pre-change organizational identification on post-change identification and various adjustment outcomes if both pathways were inaccessible, thereby contributing to employees' experience of social identity loss. Discussion focuses on the ways in which organizations and their leaders can better manage organizational change and associated identity transition. (PsycInfo Database Record (c) 2022 APA, all rights reserved).