The effect of aging over 135 days on the rate of separation of oil from 50% liquid petrolatum-50% water emulsions with both 0.2 and 0.4 sodium dodecyl sulfate (based on the water) was determined in an ultracentrifuge at 25" and 39,460 r.p.m. The concentration of sodium dodecyl sulfate in the equilibrium aqueous phase was also determined. The stability of the emulsion with 0.2% sodium dodecyl sulfate decreased for the first 25 days and thereafter increased with time, with a qualitative change in the mode of loss of oil after 135 days. There was no change in the stability of the emulsion with 0.4% sodium dodecyl sulfate over the same time period. The results with the 0.2% sodium dodecyl sulfate emulsion can be rationalized in terms of an initial increase in drop size with an accompanying desorption of sodium dodecyl sulfate and a decrease in stability, followed by coverage of a greater fraction of the remaining oil-water interface by adsorbed sodium dodecyl sulfate from the more concentrated solution of sodium dodecyl sulfate resulting from the initial desorption. Key phrases 0 Liquid petrolatum-water emulsions--effect of age on stability, ultracentrifugation 0 Stability of liquid petrolatum-water emulsions-age effect 0 Emulsions, liquid petrolatum-watereffect of age on stability, ultracentrifugation 0 Ultracentrifugation -determination of stability of liquid petrolatum-water emulsions