A method has been developed for sampling the electrolyte (concentration CD that exists during electrolysis at the interface of electrode electrolyte. Copper was electrodeposited from sulfate baths (concentration C~) on to the outside surface of a hollow, microporous metal cylinder, and electrolyte was slowly drawn into the interior which contained an organic liquid immiscible with water. The electrolyte being less dense rose to the surface of the organic liquid and was collected. The difference, Cb -- C~ ~ ACu, was somewhat greater than theoretical. Also, ~Cu was larger for baths of higher copper content. From the values of C,, the Nernst concentration polarization was calcu- lated and shown to be about 10 mv in ordinary plating operations and hence is not significant. A knowledge of the concentrations of ions in the vicinity of an electrode is necessary for a clear un- derstanding of the mechanism of electrochemical processes. The concentrations of metal-containing ions affect various phenomena, such as polarization, the structure of electrodeposits, and the composition of electrodeposited alloys. The authors were partic- ularly interested in the relation between polariza- tion and the concentrations existing at the interface of cathode and electrolyte. There has been a con- siderable amount of speculation about the magni- tude of concentration polarization. It has not been measured with certainty, because the methods that have been used were indirect and involved un- proved assumptions. The most direct method for determining concen- tration polarization would be to measure the metal ion concentration existing at the interface of the electrode and the solution and apply Nernst's for- mula to the data. Although several methods have been proposed for measuring concentrations at the interface, each one has certain disadvantages. A new method for isolating the interfacial solution has been developed and was applied to the study of the concentration polarization that occurs during elec- trolysis of copper sulfate solutions.