Research funded as part of CISSEM at Georgia Tech (Brédas, Marder), The University of Arizona (Ratcliff) and the National Renewable Energy Laboratory (Berry) combines theory and experiments, in excellent agreement, to achieve a comprehensive understanding of the energetics for a gallium‐doped zinc oxide (GZO) surface modified with five different organic phosphonic acids (PAs) to tune surface and interface properties. Density functional theory (DFT) calculations (with a repeated-slab approach) and ultraviolet photoelectron spectroscopy measurements reveal and describe changes in the density of states features at the GZO valence band edge after PA depositions. Such insight into energy level alignments of the PA molecule frontier molecular orbitals with the valence band edge and Fermi level of the GZO surface, are important for organic optoelectronic applications. The new interfacial states created by PA surface modifiers can impact charge injection or extraction with adjacent active organic layers, which can be critical to optoelectronic device performance. This excellent agreement between measured and DFT-calculated energy level alignments and density of states features is rather unusual – attributed here to the strong bonding between PAs and GZO surface zinc atoms.