Under-used and never-implemented information systems represent wasted investment. This paper proposes a new model – the Information Systems Acceptance (ISA) model - to explain and predict information systems (IS) acceptance. The ISA model integrates three sets of factors influencing IS acceptance: software characteristics; implementation processes; and system use, user satisfaction and system outcomes. These sets of factors are drawn from well-established frameworks: Rogers' (1995) Diffusion of Innovations (DOI), Land's (1994) Guidelines for Successful Implementation of Information Systems, and DeLone and McLean's (2003) Model of Information Systems Success (ISS). By building on established theories, the ISA model incorporates and organises previous research, overcomes some of the limitations associated with these individual theories, and assembles IS acceptance factors into a comprehensive, understandable and useable conceptual framework. The ISA model gives managers a tool to evaluate and predict the likely success of an IS, and acts as a diagnostic aid in preventing potential problems. We demonstrate the ISA model by using it to explain acceptance levels of a quality systems software application implemented in ten organisations.