The adsorption and subsequent dissociation reaction of methanol on Pd (111) has been the focus of several studies due to the importance of this reaction as a model for the production of methanol from hydrogen and carbon monoxide. While the question, whether the C-O bond of adsorbed methanol on Pd (111) can be opened, has been settled recently, there is no experimental proof for the mechanistic explanation of this reaction pathway. By combination of thermal desorption spectroscopy and density functional theory we present evidence that a hydrogen bond between two neighboring methanol molecules on the Pd (111) surface is necessary to break the C-O bond. A complete suppression of the C-O bond cleavage in methanol is possible by changing the electronic structure of the surface due to the formation of a Pd/V surface alloy. The same result can be achieved by using a molecular beam of methanol with a high translational energy. This effect is explained in terms of dissociative adsorption via C-H bond scission at high methanol energies instead of molecular methanol adsorption.