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Catalytic reactions in the unstirred layers near bilayer lipid membranes can induce nonmonotonic concentration profiles near the membrane surface. In the case of transmembrane diffusion of a substrate immediately followed by its conversion due to the presence of an aqueous soluble enzyme the size of the unstirred layer defined in terms of the concentration gradient at the membrane surface does not correspond to the width of the! aqueous layer adjacent to the membrane where the concentration differs from the bulk phase concentration. Deducing of flux values or convection parameters from the concentration gradient at zero distance from the membrane gives misleading results. An empirical equation for the estimation of the size of the concentration boundary layer is proposed. It was derived from pH profiles registered with the help of a microelectrode near a planar bilayer lipid membrane surrounded by a buffer solution containing at one side of the membrane aa;taldehyde and sodium acetate and at the other side alcohol dehydrogenase. Since this parameter equals to the thickness of the unstirred, layer in the case of exponential concentration profiles it may be applied to estimate both mass transfer restrictions and kinetic of diffusion limited reactions occurring in the immediate membrane vicinity regardless the complexity of the system under investigation.