Fresh natural rubber latex particles are stabilized by adsorbed proteins and phospholipids while those for high-ammonia latex concentrate are stabilized mainly by adsorbed long-chain fatty acid soaps, the hydrolysis products of phospholipids. The structure of this surface layer surrounding the latex particles is analyzed using the electrophoresis model of Ohshima and Kondo. Good agreement between theoretical and observed electrophoretic mobility was found, except at very low ionic strength. The results were consistent with a model of an uncharged bared polyisoprene latex particle coated by a charged mixed layer of long-chain fatty acid soaps, proteins, and polypeptides of finite thickness in matured latex concentrate. This surface layer is ion-penetrable and its resistance to liquid flow changes with time on prolonged hydrolysis.