We have investigated the coadsorption of perylene tetracarboxylic dianhydride (PTCDA) and tetraaminobenzene (TAB) on the Ag/Si(111)-root 3x root 3 R30 degrees surface using scanning tunneling microscopy. At room temperature, PTCDA islands with square and herringbone ordering are formed which, on exposure to TAB, are converted into an intermixed phase in which PTCDA and TAB form alternating rows. From our images, we determine the relative placement of TAB and PTCDA molecules and conclude that the row structure is stabilized by hydrogen bonding between dianhydride and diamine groups. We confirm that this hydrogen bonding junction is stable using ab initio calculations and show that the proposed geometry is consistent with calculated intermolecular dimensions.