A method of sampling the film of electrolyte immediately adjacent to a plating electrode has been developed. A small hole was drilled through the electrode, and electrolyte slowly siphoned through this hole while the plating operation was in progress. Experimental results are presented for the acid copper sulfate plating bath. The copper decrease and acid increase in the cathode film were studied under various conditions of current density, copper concentration, acid concentration and temperature. Variations in acid concentration were found to affect the film more than changes in copper concentration. An increase in temperature markedly decreases the difference between the film and main body concentrations. The ratio of copper to acid in the film has been considered, and the results interpreted from this point of view. The effect of the location of the sampling hole on the electrode on the composition of the film was also studied. On moving the sampling hole horizontally from the middle to the edge of the electrode, little difference in the composition of the film was noted. Large differences in the film composition were found when the samples were taken from the bottom, middle and top of the electrode, even though the electrolyte was sufficiently well stirred to avoid stratification.