The monolayer crystal structure of phenazine adsorbed on graphite is determined by a combination of synchrotron X-ray diffraction and DFT calculations. The molecules adopt a rectangular unit cell with lattice parameters a = 13.55 Å and b = 10.55 Å, which contains 2 molecules. The plane group of the unit cell is p2gg, and each molecule is essentially flat to the plane of the surface, with only a small amount of out-of-plane tilt. Density Functional Theory (DFT) calculations find a minimum energy structure with a unit cell which agrees within 7.5% with that deduced by diffraction. DFT including dispersion force corrections (DFT+D) calculations help to identify the nature of the intermolecular bonding. The overlayer interactions are principally van der Waals, with a smaller contribution from weak C-H•••N hydrogen bonds. This behaviour is compared with that of 4,4’-bipyridyl.