The aggregation of porphyrins is a well-known behavior of this class of com
pounds. Aggregated porphyrins play a key role in man!: systems related to m
edicine and biology, e.g., the bacteriochlorophyll dimer, known as the "spe
cial pair", is the main electron donor in the bacterial reaction center. Th
e large surface and planarity of the porphyrin chromophore favor strong van
der Waals stacking interactions which lead to very large aggregates. Recen
tly, we have studied the aggregation of a synthetic water soluble porphyrin
(trans-H2Pagg), which on salt addition gives mesoscopic aggregates having
a fractal structure mainly driven by a diffusion limited kinetic (DLA). Her
e we are reporting a study of such aggregation property on varying the salt
concentration, obtaining evidences that differ ent dynamics of aggregation
exist in different ionic strength regimes.