Surface active betaine esters contain a hydrolysable bond and give naturally occurring products (fatty alcohol and the amino acid betaine) on degradation. They are therefore interesting candidates for use as cationic surfactants in pharmaceutical applications. In this work the phase behavior of two systems of relevance for the utilization of dodecyl betainate as a pharmaceutical excipient is studied, namely dodecyl betainate/dodecanol/betaine hydrochloride/D2O and dodecyl betainate/phosphatidyl choline (PC)/ethanol/D2O. The techniques used for phase characterisation were H-2 NMR measured on the solvent, small angle X-ray spectroscopy and optical microscopy. Dilute dodecyl betainate/PC dispersions were characterized using laser diffraction. It is shown that introduction of relatively small amounts of the hydrolysis products of dodecyl betainate, i.e., dodecanol and betaine (used in the form of betaine hydrochloride), has a strong effect on the phase behavior of the binary dodecyl betainate/D2O system. The degradation products change the average curvature of the surfactant film so that, instead of a hexagonal phase at concentrations above the micellar phase, a probably defective, lamellar phase seems to form. The dodecyl betainate/PC/ethanol/D2O system shows a large region of a highly swelling lamellar phase. Dispersions of dodecyl betainate/PC/ethanol in water can be prepared with low energy input; i.e., the preconcentrate can be regarded as a self-dispersing solution. Introduction of dodecyl betainate and its degradation products does not impair the ability of PC to form vesicles. Experiments for evaluating the toxicity of surface active betaine esters to erythrocytes were also performed. There are indications that the hemolytic activity of dodecyl betainate is lower than that of the stable surfactant tetradecyltrimethylammonium chloride, which has similar critical micelle concentration. A combination of dodecyl betainate and PC gives very low hemolytic activity. (C) 2004 Elsevier Inc. All rights reserved.