The interaction of a class of oxonol dyes with sonicated phospholipid vesicles was followed optically. The spectra of vesicle-associated dyes resemble those found for the dyes in organic solvents, indicating that the oxonols occupy a hydrophobic region of the membrane. At equilibrium the affinity of the oxonols for the vesicles depends on the structure of the dye, the physical and chemical composition of the vesicles, and the ionic strength of the medium. The oxonols occupy soybean lipid vesicles to a level of 147.9 +/- 17.1 nmol/mg lipid with a dye membrane dissociation constant of 3.33 +/- 0.54 muM. The interaction of the oxonols with soybean lipid vesicles is biphasic. The fast phase has a second order rate constant of 9.04 +/- 0.36 x 10(6)M(-1) s(-1) and the number of "fast" binding sites, 68 +/- 8 nmol/mg lipid, was determined from the ratio of the second order rate constants obtained with lipid and with dye in excess. The dissociation of oxonols from soybean lipid vesicles is also biphasic, and the fast process has a rate constant of 17 +/- 2 s(-1), yielding a dissociation constant for the fast sites (k(-1)/k(2)) of 1.88 +/- 0.15 muM. The slow phases of oxonol association with, and release from, soybean lipid vesicles are not second order and have half times of between 0.2 and 5 min, depending on the physical and chemical composition of the membrane lipids. The amplitudes of the slow phases are sensitive to the composition of the aqueous media on each side of the vesicle membranes, which suggests that the slow processes represent the permeation of the membrane by the oxonols. The importance of the properties of the oxonol dyes in the interpretation of their behavior in natural membranes is discussed.