Aggregates of the diacid form of tetrakis(4-sulfonatophenyl)porphine (TPPS), formed in acidic aqueous solutions, were deposited onto hydrophilic (mica) and hydrophobic (polystyrene and graphite) substrates and imaged using atomic force microscopy (AFM). The AFM images revealed the aggregates were either individual rods with a diameter of 3.8 +/- 0.3 mn and a length of 0.77 +/- 0.43 mum or larger structures composed of bundles of individual rods. This study demonstrates that aggregates of the diacid form of TPPS are remarkably straight nanorods with a well-defined height. UV/vis spectroscopy and dynamic light-scattering measurements show that the aggregates form in solution and not on the surface of the substrate, with nucleation of the aggregates being the rate-limiting step, followed by rapid growth at the ends of the rods.