Although some authors have stated that effective IT governance is crucial for any organization to achieve its corporate goals, little empirical research is available supporting the assumptions regarding the factors that determine the effectiveness of IT governance. This paper analyzes the main IT governance domains (i.e., IT strategic alignment, IT risk management, IT value delivery, IT resource management, and IT performance management) and the presence of several IT governance mechanisms that constitute antecedents of IT governance effectiveness. We used partial least squares (PLS) structural equation modeling to test our hypotheses based on survey data from 87 large Brazilian companies. Our results show IT strategic alignment, IT value delivery, IT risk management, and IT performance management have a positive and significant impact on the effectiveness of IT governance. Further, we found associations of structural, procedural, and relational mechanisms with the main IT governance domains suggesting their adoption can improve IT governance within organizations.