Liposomes of different sizes are frequently used model systems for cellular membranes. To mimic the cellular environment, these liposomes are often prepared from a mixture of different lipids in organic solution. The preparation involves, at some point, the transfer into aqueous solution. Thus, both the total amount of lipid and the relative amount of each lipid species might deviate from the original composition in the organic solvent. We used thin-layer chromatography combined with a lipid extraction step to check whether the liposomes in the final aqueous solution have the intended composition. This allows determination of the lipid composition not only for large unilamellar vesicles, but was also applied, for the first time, to giant lamellar vesicles (GUVs), which typically are available only with a low amount of lipid per preparation. For different ternary and quaternary mixtures of cholesterol, phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, and sphingomyelin, the final composition agreed within 15% with the intended composition in most cases, but in certain cases, such as GUVs prepared with a large fraction of phosphatidylethanolamine, the deviation can be significant. This shows that in those cases where the composition plays an important role, it is advisable to check the final composition of these model membranes.