In this work, a systematic study of effects of three cosolvents (methanol, ethanol, and 1-propanol) on the stability of liposomes formed from soybean phosphatidylcholine (PL-90) by a semispontaneous process was carried out. The experimental results revealed that significant enhancement of PL-90 liposome stability could be achieved by cosolvent addition with suitable amounts. A similar phenomenon was also observed for catansomes formed from ion-pair amphiphiles (IPAs) as demonstrated by decyltrimethylammonium dodecyl sulfate (DeTMA-DS) with a comparatively high concentration of 5 mM. In general, with continued increase in the cosolvent concentration, the stability of liposomes and catansomes first increased, reached a maximum, and then decreased. Furthermore, it was realized that the cosolvent effects on the liposome stability were very similar to those on the catansome stability, which might be also explained by the mechanism proposed on the basis of the viewpoint of a mixed solvent dielectric constant.