Protolytic and partition equilibria of a series of indicator dyes (Neutral Red, Acridine Orange, Bromothymol Blue, Bromophenol Blue, Methyl Yellow, Methyl Orange, Hexamethoxy Red, Quinaldin Red) were studied in a suspension of liposomes composed of phosphatidylcholine and diphosphatidylglycerol (18:1, mol:mol). The ‘apparent’ pK_a^a values were determined spectrophotometrically for various phospholipids (up to 3 mmol dm^-3) and supporting electrolyte (up to 2 mol dm^-3) concentrations. The calculated binding constants (K) of various dyes allowed extrapolation of pK_a^a to pK_a^ac, characterizing the acid–base properties of completely bound dyes. This revealed some solvating and electrostatic properties of the lipid–water interface, such as that the microenvironments of Neutral Red, Acridine Orange and Bromothymol Blue in the lipid bilayer are intermediate between those of non-ionic and anionic micelles, while the pK_a^ac values of lipophilic indicators are similar to those observed in micelles of anionic surfactants. In the acidic region (pH<3) the medium effects (ΔpK_a^ac=pK_a^ac-pK_a^w) are similar to those observed with non-ionic micelles, as seen for Methyl Yellow and some other dyes. This is in agreement with the fact of protonation of the lipid's phosphate groups.