In repeated transport experiments through organic supported liquid membranes (SLMs), the decrease of the cation permeability coefficient "P-M", as defined in Danesi’s thermodynamic model, has been chosen to characterize membrane stability. A simple linear relation between log P-M and the number of runs (i - 1) was derived from the model and verified on several different experimental cases of selective coupled co-transport of metallic cations. SLMs contained either organophosphorous compounds or macrocycles such as classical crown-ethers or calixcrowns. All cases have demonstrated the validity of the proposed relation in as much as the physical characteristics of the SLM remained constant. Unfortunately, by leaching the SLM continuously, the daily renewed contacting aqueous solutions dissolved both the organic diluent and the carrier, leading to SLM instability. Some deviations thus occurred after many runs of repeated transport experiments (i > 20 in some cases). Nevertheless the mathematical relation reported in this paper allowed the apparent partition constant of the carrier to be experimentally estimated, thus permitting the hydrophobicity and efficiency of different carriers to be compared.