The aggregate formation of p-hydroxybenzoic acid esters (parabens) in aqueous solutions was studied by ultrafiltration and light scattering. The permeability of parabens in aqueous solution passing through an ultrafiltration membrane decreased with an increase in carbon number in the alkyl group from methyl- to butyl-paraben. Dissociation of the phenolic hydroxyl group and the addition of ethanol made possible the free passage of butyl-paraben through the membrane. An increase in the concentration of butyl-paraben was responsible for the steep rise in light scattering intensity plots. Undissociated molecules of parabens in aqueous solution are thus shown to form aggregates by hydrophobic intermolecular force. The membrane permeability of butyl-paraben decreased and increased with concentration and the cutoff molecular weights of the membranes, respectively. Plots of surface tension against concentration of butyl-paraben showed a linear decrease but no inflection point. Paraben molecules possessing long alkyl chains may thus be concluded to form non-micellar type aggregates in aqueous solution, which differ from the clear-cut aggregate formation of conventional surfactants.