The oxidation of a blend of model molecules (tetralin plus indane in hexadecane) and two deep hydrotreated LCOs was performed by treating the fuel in the presence of air with a CuCr/P(4-PVP) catalyst. The blend was studied at different operating conditions, and the product composition was determined by elemental analysis, and by GC. The hydrotreated LCO were fractionated in cuts, and the naphtheno-aromatic compounds present were characterized by HPLC, GC-MS, and HNMR techniques, as well as elemental analysis, and ASTM methods. These cuts were oxidized in a continuous stirred tank reactor from which samples of product were taken at different residence times and analyzed. In particular, the acid number, storage stability, and cetane number were followed as a function of the oxidation severity. The oxidized products were tested in a diesel engine. Their emissions were compared with those of a biodiesel, a blend, and a hydrotreated product. The results indicated that the oxidation rate and the type of molecules are different in the light, medium, and heavy part of the hydrotreated products, with a different acid number, cetane number storage stability, and emissions in a diesel engine throughout the distillation range. The paper discusses the mechanism of the oxidation reactions, the effects of severity in the hydrotreating, and the effect of oxygenated molecules contained in the light part of the diesel.