We have developed electrochemical porosimetry (ECP) for analyzing microstructures of porous electrodes in situ, which can give most meaningful geometric information in electrochemical systems. The methodology is based on a model that relates electrochemical impedance data with microstructural information. Pore length (l(p)), as well as pore size distribution could be obtained by fitting the model to the experimental impedance data of a porous electrode. This geometric information was validated for microporous, mesoporous, and macroporous samples. Also, the ECP could be used as a nondestructive probe to investigate the construction of electrochemical devices. (C) 2004 The Electrochemical Society.