In today's corporate environment, most enterprise-wide information systems are used in mandatory (non-voluntary) settings. In such environments, unfavorable user acceptance behavior has been identified as one of the significant causes of failure in successful system implementation. Under the recognition that appropriate expectation management is one approach to determining favorable system acceptance attitudes, this study empirically analyzed the effects of outcome expectations on user acceptance behavior. Additionally, to overcome the limitations of system usage intention and system usage—which have been traditionally employed—being inadequate in mandatory system usage environments, this study proposed system usage activities as a proxy variable for actual usage that can explain substantive acceptance behavior. The structural equation model analysis targeting ERP system users confirmed the significant effects of personal outcome expectations and user satisfaction on information system usage activities. The results of this study are expected to contribute to understanding user acceptance behavior in mandatory information system usage environments.