It is commonly believed that organic molecules are physisorbed on the ideal nonpolar surfaces of wide band gap insulators with limited variation in the electronic properties of the adsorbate molecule. On the basis of first principles calculations within density functional theory (DFT) and GW approximation, we show that this is not generally true. We find that the molecular frontier orbitals undergo significant changes when a hydroxy acid (here we chose gluconic acid) is adsorbed on MgSO4 H2 O (100) surface due to the complex interaction between the molecule and the insulating surface. The predicted trend of the adsorption effect on the energy gap obtained by DFT is reversed when the surface polarization effect is taken into account via the many-body corrections. © 2010 American Institute of Physics.