The dataset consists of data on 5 experiments assessing the combined influence of Cu and Ni on North Sea Mytilus edulis larvae. All experiments are identical apart from the assessed concentrations. The experiments were performed according to the ASTM E724-98. Mussel embryos were exposed to different concentrations of Cu and/or Ni for 48 h. Thereafter the development of the larvae was assessed via a microscope to define the toxicity of the metal mixture. The data acquisition and statistical analysis and results are described in detail in Deruytter, D.; Baert, J.M.; Nevejan, J.; De Schamphelaere, K.A.C.; Janssen C.R. (2017) Mixture toxicity in the marine environment: model development and evidence for synergism at environmental concentrations. Environmental toxicology and chemistry.