The mechanism of adsorption of the 1,4,5,8-naphthalene-tetracarboxylic-dianhydride molecule on the Ag(110) surface is elucidated on the basis of extensive density functional theory calculations. This molecule, together with its perylene counterpart 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA), are archetype organic semiconductors investigated experimentally over the past 20 years. We find that the bonding of the molecule to the substrate is highly site selective, being determined by electron transfer to the lowest unoccupied molecular orbital of the molecule and local electrostatic attraction between negatively charged carboxyl oxygens and positively charged silver atoms in [1(1) over bar 0] atomic rows. The adsorption energy in the most stable site is 0.9 eV. A similar mechanism is expected to govern the adsorption of PTCDA on Ag(110) as well.