Abstract: A process for preparing large unilamellar lipid vesicles is disclosed which process comprises preparing unilamellar lipid vesicles having an average diameter of at least 1,000 .ANG., each of these vesicles consisting of a spheroidal lipid lamella containing a substance to be encapsulated, which process comprises dissolving one or more lipids intended to form the lamella of the vesicles in at least one water-insoluble solvent, conditioning the resulting lipid solution at a pressure P.sub.1 and at a temperature .theta..sub.1, dissolving the substance to be encapsulated in water to obtain an aqueous phase, conditioning said aqueous phase at a pressure P.sub.2 and a temperature .theta..sub.2, injecting the lipid solution into the aqueous phase at a low flow rate so as initially to form droplets and so that solvent of the lipid solution vaporises on coming into contact with the said aqueous phase, the pressure P.sub.2 being less than P.sub.