Pseudo isocyanine chloride (PIC) has a strong tendency to self-assemble with a concentration dependent temperature threshold separating a regime with small H-oligomers in equilibrium with monomeric PIC from a regime where large J-aggregates form. In complementing the known set of absorption spectra by the spectrum of a trimer, which represents all H-aggregates with N ≥ 3, a full description of the sample composition of PIC in the regime of oligomers became possible by means of UV-vis spectroscopy and gave access to the equilibrium thermodynamics of oligomerisation. Successful interpretation of the concentration dependent temperature threshold as a ceiling temperature of J-aggregation made also accessible equilibrium thermodynamics of the formation of J-aggregates together with a full analysis of composition also in the regime of J-aggregates. Evolution of an invariant spectrum for the J-aggregates demonstrates full consistency of the composition analysis. Complementary light scattering experiments led to a comprehensive characterisation of all aggregate species above and below the aggregation threshold. Once initiated, J-aggregates always grow to a final length of 650 nm. Time-resolved light scattering confirmed that the self-assembly of J-aggregates follows a monomer addition process in analogy to a chain growth in polymer chemistry. Initiation and growth of individual aggregates turned out to be always much faster than the progress of aggregation.