An ongoing chemical and ecotoxicological study of Water Accommodated Fraction of oils is presented and the preliminary findings are discussed. The study aims at obtaining improved and realistic data on potential environmental effects of various oils released and weathered at sea. Such data will be used for improving algorithms in present fate and effect models for damage assessment studies and "Net Environmental Benefit Analysis" of response alternatives in various spill scenarios, Preliminary results show that models used to assess effects in the mater column will need to resolve the water soluble fraction of oils into more than one single bulk parameter to produce realistic estimates of effects,