The strategic use of different human resource management (HRM) systems for different employee groups within the same organization, such as use of a high investment HRM system toward core employees and a more control- oriented HRM system toward noncore employees, is not uncommon, and is one means of allocating scarce resources while supporting strategic objectives. However, current theory has not fully explored the consequences of strategic HRM system heterogeneity at the employee level, so we know little about the mechanisms through which such heterogeneity might ultimately achieve (or fail to achieve) its intended aims. How system heterogeneity is perceived and how employees react to it is a central concern. In this paper, we focus our attention on employees’ reactions as manifested in the motivation to engage in voluntary behaviors. We develop a theoretical model of how strategic heterogeneity in HRM system use is perceived by employees in different groups and how it affects the patterns of employees’ organizational citizenship behaviors toward core employees, noncore employees, and the organization. This paper also explores individual differences and contextual factors that may moderate these relationships. We highlight the important implications of our model for scholars conducting strategic human resource and strategic human capital research, as well as for managers of organizations utilizing heterogeneous HRM systems.