Abstract: A method for the determination of the morphological integrity of a membrane of lipid vesicles, such as liposomes, using electron spin resonance (ESR) spectroscopy including the steps of: a) labeling the lipid vesicles with an ESR-active probe; b) producing a sample by introducing a quantity of the labeled lipid vesicles into a test medium; c) producing a positive control by introducing a quantity of the labeled lipid vesicles into a control medium and optionally a negative control by introducing a quantity of the ESR-active probe into the test medium; d) obtaining ESR spectra of the controls and the sample; and e) comparing ESR spectra of the sample and controls to determine relative morphological integrity. Morphological integrity of lipid vesicles may be quantitatively determined in the test medium by obtaining difference spectra produced using the spectra of the sample and of the positive and/or negative control.