This dataset provides concentration-response data and associated general chemistry conditions for 26 experiments consisting of 149 tests regarding the acute toxicity of major ions to Ceriodaphnia dubia in a variety of test waters; it also provides LC50 estimates and the estimated ion mixtures at the LC50 for each toxicity test. This dataset is associated with the following publication: Mount , D., R. Erickson , T. Highland , R. Hockett , D. Hoff , T. Norberg-King , K. Peterson, Z. Polaske, and S. Wisniewski. The acute toxicity of major ion salts to Ceriodaphnia dubia: I. Influence of background water chemistry. ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. Society of Environmental Toxicology and Chemistry, Pensacola, FL, USA, 35(12): 3039-3057, (2016).