In order to develop water-soluble porphyrins, alkyloxo(methoxo)porphyrinatoantimony bromides (alkyl = hexyl (1a), decyl (1b), dodecyl (1c), tetradecyl (1d), octadecyl (1e)) were prepared. 1 had more than 1 mmol dm(-3) of solubility in water. From the dependence of the half-width of the bands in the absorption spectra and surface tension on the concentration of 1, it was estimated that 1b-d were present as aggregates in concentrations higher than 10 micromol dm(-3). From the NMR analysis in D(2)O, it was deduced that the alkyloxo ligands of 1 were arranged alternately in the aggregates. The diameter of the aggregates of 1 in water was determined to be around 100 nm by the dynamic light scattering method. Since the solubilities of di(methoxo)tetraphenylporphyrinatoantimony bromide and 5-(4'-decyloxyphenyl)-10,15,20-triphenylporphyrinato(dimethoxo)antimony(v) bromide were low, it was calculated that the long alkyl axial ligands were requisite for the high solubility in water.