The spectroscopic aggregation number (coherence length) and physical size of the J-aggregates of anionic mesotetrakis(4-sulphonatophenyl)porphyrin (H4TPPS2-) are controlled by ammonium ion concentration. The contributions of absorption and scattering are determined using the extinction and resonance light scattering (RLS) spectra of the TPPS J-aggregate. The highest spectroscopic aggregation number (12.9) was determined for the J-aggregate of hydrodynamic radius of 56 +/- 10 nm. (C) 2003 Elsevier Science B.V. All rights reserved.