The spectral properties, cis --> trans isomerization and aggregation of the anionic hydrophilic cyanine 1, were studied in isooctane/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/water, cyclohexane-hexanol/Triton X-100/water reverse micelles and in aqueous solutions in the presence of surfactants. The incorporation of I into micelles leads to dimer --> monomer decomposition and cis trans isomerization. In AOT reverse micelles with w(0) greater than or equal to 5 (w(0) = [H2O]/[AOT]) 1 forms J aggregates at concentrations much lower than one dye molecule per one micelle; at w(0) = 50 the most probable aggregation number is 3. Premicellar J aggregates of 1 are observed with both cationic and anionic surfactants, but not with Triton X-100.