Molecular n-type doping of 1,4,5,8-naplithalene tetracarboxylic dianhydride (NTCDA) by pyronin B (PyB) is investigated using ultraviolet photoelectron spectroscopy (UPS), inverse photoelectron spectroscopy (IPES). and current-voltage (I-V) measurements. Deposition of small amounts (< 2 angstrom) of PyB on pristine NTCDA films leads to a shift of all the molecular levels away from the Fermi level by nearly 0.20 eV, indicative of n-type doping of NTCDA by PyB. Interface and bulk energy levels of films formed by co-evaporation of host and dopant show similarly efficient n-doping. The spectroscopic measurements are confirmed by I-V measurements, which show a four-orders-of-magnitude increase in current in doped films. The comparison of data obtained from UPS of the neat PyB film with the results of density functional theory calculations confirm that two species of PyB are evaporated and condensed into the solid state. with one species primarily responsible for doping.