Perfluorocarbon macroemulsions were stabilized by phospholipids which were composed mainly of phosphatidylcholine and phosphatidylethanol amine. Emulsions which were prepared with a phosphate buffer as the external phase, were more stable than those prepared with water only, in spite of the very high ionic strength (I=0.4) in the former. It was found that hydrolysis of the phospholipids caused formation of lysophosphatidylcholine, and a significant decrease in the pH, in absence of the buffer. The decrease in pH led to a decrease in zeta potential, and hence decreased stability. This effect was not significant in the buffer-emulsions. However, oxidation of the surfactant was detected for both systems, a process which resulted in oil separation after prolonged time (>200 days) also in the oil in buffer emulsion.