We present the first quantitative proof of the diffusive character of the mass transport in thin gap electrodeposition experiments. Applying phase-shift interferometry for the local determination of the concentration field, we demonstrate that the diffusion equation can accurately describe the mass transport in cells whose gap is less than 50 mm. We also show the correspondence of the time evolution of experimental interfacial depletion profiles along the reacting electrode with the theoretical prediction of a simple diffusion equation.[S0031-9007 (97)03477-7] PACS numbers: 81.15.Pq, 61.43.Hv, 68.35.Rh, 81.10.MxSince the beginning of this century, diffusion-limited interfacial processes have been the object of strong interest among electrochemists and physicists because they can provide sufficiently simple transport equations to enable some theoretical modeling [1][2][3][4]. In particular, the electrodeposition of metals has been the subject of multiple theoretical and experimental studies.In the early eighties, the characterization of the fractal morphology has been used as a test for recognition of diffusion-limited aggregation processes [5]. In a more recent work, it has been possible to find a strategy to bring the electrodeposition system in the "optimum conditions" for recovering this limit [6,7]. However, in all cases, the experimental argumentations which were invoked to ensure that the growth process was diffusion-limited remained phenomenological. On the other hand, the characterization of the morphology and growth probability distribution [8] of electroless (displacement of metals) deposits and their comparison with numerical off-lattice DLA aggregates yield only qualitative indication concerning the nature of the mass transport involved in the growth.In this paper, our approach is definitely a step beyond, since we bring the first local proof of the occurrence of diffusion-limited transport processes in electrodeposition experiments. This implies several experimental requirements. The first one is to work with very thin gap cells (50 mm), to get rid of buoyancy-driven convection [9,10]. Furthermore, the precision and the resolution of the Mach-Zehnder interferometer [11] that we have used previously [12] to measure concentration fields must be improved. The phase-shift interferometry (PSI) [13 -15] answers to this requirement. After a brief description of the experimental technique, we show that the experimental concentration field fits a diffusion equation when the gap of the cell is less than 50 mm. This result is further confirmed by the comparison of the time evolution of the concentration profiles with the theoretical predictions of a onedimensional diffusion equation.A sketch of the experimental setup is given in Fig. 1. The output of a 15 mW TEM 00 He-Ne laser is spatially filtered and split by a beam splitter (BS1) into two arms. The half wave plate QW adjusts the polarization of the beam in order to enhance the fringe contrast. The mirror M1 is mounted on a piezoelectric crystal (PZT) and it...