This study investigated through empirical methods the extent to which information acquisition and information management capacity of administrators in tertiary institutions in South-Western Nigeria contributed to their decision-making effectiveness. It adopted the ex post facto survey research design, using the random sampling technique to select 1,357 out of the 1,799 administrators in tertiary institutions in Ekiti, Lagos and Oyo states of the region. The One-Way Analysis of Variance and Multiple Regression Analysis were used to determine the composite impact of information acquisition and information management on administrators' decision-making effectiveness, while the Stepwise Multiple Regression was used to determine the relative contribution of each of information acquisition and information management to administrators' decision-making effectiveness. The 0.05 level of significance was chosen for the acceptance or rejection of the research hypotheses. The findings revealed that information acquisition and information management capacities whether taken separately or jointly make significant contributions to the decision-making effectiveness of administrators in tertiary institutions in the region (0.291 and 0.494 respectively for relative contribution, and 0.478 for composite contribution, p < 0.05). Hence, the study recommended modernization of information systems in these institutions with a view of improving information acquisition and information management capacities of the administrators.