The organization of an amphiphilic azobenzene derivative, 4-octyl-4'-(5-carboxy-pentamethyleneoxy) azobenzene (8A5), in monolayers at the air-water interface is strongly controlled by association phenomena due to intermolecular interactions of the azobenzene moiety. The monolayer behavior has been investigated by measuring surface pressure-area and surface potential-area isotherms as well as by Brewster angle microscopy (BAM) and reflection spectroscopy. In particular, dimers are formed that reorient from a flat or slightly tilted orientation to an orientation nearly normal to the water surface and forming H-aggregates upon monolayer compression. The BAM images clearly show domains with long range tilt orientational order. The observed shifts of the reflection maximum as well as the intensities of the band are accounted for by a model of molecular association and reorientation. The intermolecular interactions are strongly modified by variation of the subphase pH controlling the dissociation of the hydrophilic carboxyl group. On a subphase pH = 9.4, flat lying monomers are observed at low surface pressure and dimers form upon compression, however, no domains are detected by BAM in this case.