The periodic extraction of a solute from an aqueous solution into a thin layer of an organic solvent (extractant) and its back extraction into a second aqueous solution have been studied experimentally. An aqueous solution of citric acid, whose temperature was cycled as a square wave, was circulated through a bed of polypropylene pellets that were coated with a thin layer of an organic solvent containing Alamine 336. The results indicate the following: (a) Periodic extraction by means of a minute amount of extractant, spread on a suitable support material, is feasible. (b) Frequent shallow penetration of the solute into the extractant layer is both feasible and useful as it will intensify the overall average mass transport. (c) The penetration by diffusion of the solute into deep layers of the extractant is too slow to affect the average overall mass transfer. The results of this study should be applicable to liquid-liquid extraction systems using a driving force other than thermal.