Acetylsalicylic acid (aspirin) suppresses the generation of prostaglandin H2, which is the precursor of thromboxane A2. Aspirin acts as an acetylating agent in which its acetyl group is covalently attached to a serine residue (S530) in the active site of the cyclooxygenase-1 enzyme. The exact reaction mechanism has not been revealed by experimental methods. In this study the putative structure of human cyclooxygenase-1 was constructed from ovine cyclooxygenase-1 by homology modeling, and the acetylsalicylic acid was docked into the arachidonic acid binding cavity of the enzyme. To characterize the shape of the potential energy surface of the acetylating reaction and to determine the relative energies of the stationary points on the surface, a series of ONIOM-type quantum mechanical/molecular mechanical (QM/MM) calculations were carried out at different QM levels of theories applying electronic embedding approximations. The acetylsalicylic acid and the surrounding amino acids were included in these calculations. Frequency analyses were performed to prove the existence of first order saddle points (representing transition states) and local minima on the potential energy surface. It was found that all levels of theories predicted similar transition state geometries. The activation energy values, however, demonstrated significant dependence on the methods that were applied. All the applied "dependable" ab initio and DFT methods predicted that the breakage of the S530 Oγ--Hγ and formation of the Oγ--C(acetylsalicylic acid carbonyl) bonds occur in a single elementary step.