Uncertainty assessment is critical to inform confidence in risk conclusions and guide risk-based decision making. This case study quantifies the impact of uncertainties in the United States Environmental Protection Agency (EPA)’s chronic non-cancer risk evaluation of diisononyl phthalate (DINP) under the Toxic Substances Control Act and provides critical information regarding confidence in risk determinations and risk management decisions. Uncertainties associated with key elements in EPA’s risk determination approach (i.e., margin of exposure [MOE]) were qualitatively and quantitatively examined, including point of departure (POD) selection, exposure estimates, and benchmark MOE (bMOE) calculation, to demonstrate their impact on MOEs and implications for risk-based conclusions. There was medium overall confidence in the risk evaluation, with lowest confidence in POD selection. Quantitative uncertainty assessment for each exposure scenario for which EPA determined unreasonable risk (i.e., MOE<bMOE of 30) considered a range of alternative values per key element. For the lowest MOE calculated by EPA (2.7 = unreasonable risk), alternative MOEs derived herein ranged from 117-150 (no unreasonable risk), demonstrating the uncertainty imparted in EPA’s determinations of unreasonable risk. This uncertainty assessment illustrates the importance of applying qualitative and quantitative approaches to assess uncertainty to frame the confidence in assessments that underlie risk-based decisions.