Organizations deal with complex information systems innovations such as enterprise resource planning systems to enable and support their operations. While there is considerable research on organizations’ adoption, implementation, and use of such complex information systems, prior literature has not dwelt as much on the diffusion or the spread of such complex information systems across a population of organizations. A limited number of studies have shown different information sources such as external, internal, and mixed influences to drive diffusion, and found variations in the diffusion patterns of different complex information systems. These findings, however, belong to different populations and do not account for organizational or technology characteristics that may be influential in diffusion. This study seeks to expand our understanding by examining the diffusion of several complex information systems within the same population of S&P-500 organizations between 1990 and 2008 by modeling different influence mechanisms and employing event-history analysis.