Crude oil is composed of mixtures of chemicals, which differ between locations and wells. Toxicity of crude oil differs, also depending on its composition. Weathering of oil and its biodegradation, may alter its toxicity,which ,however, may differ due to its composition. The complex mechanisms of the resulting toxicity of weathered oil is therefore not entirely resolved. To better understand the factors driving the toxicity of oil of different composition during weathering, this study will mechanistically characterize different fractions of fresh and weathered crude oil and assess the effects on a model marine benthic organism, Cockle (Cerastoderma edule).