Porosity is an important parameter by which to evaluate membrane performance. In recent years, membranes formed in microfluidic chips using laminar flow-based interfacial reaction technology have gained extensive attention due to their potential in micro-separation applications; however, so far, there has not been a feasible method to determine the porosity because of the very tiny size of this type of membrane. Here, we proposed a solute transport-based method to estimate the porosity of a chitosan-carbon nanoparticle membrane fabricated in a microdevice. In this method, a counter-current microdialysis mode was designed to experimentally study urea transport across the membrane, and a fluorescein was added to the urea solution to facilitate the observation of the urea concentration profile. Then, a porosity-related mass transfer model was developed to theoretically simulate the urea transport. The membrane porosity was obtained by iterating the porosity distribution to minimize the difference between the theoretical and experimental results. Finally, the membrane porosity was experimentally validated through a creatinine clearance study. The averaged porosities of the on-chip chitosan-carbon nanoparticle membrane formed in 15, 20, and 25 min were 0.356 +/- 0.050, 0.242 +/- 0.018 and 0.235 +/- 0.009, respectively. The method proposed here is of significance for characterizing the performance of on-chip fabricated membranes. (C) 2016 Elsevier Ltd. All rights reserved.