The aim of the investigation is to determine and compare the basic characteristics of the pores in battery separators using mercury porosimetry, which measures the volume of mercury penetrating into the pores, capillary flow porometry, which measures the flow rate passing through the pores, and scanning electron microscopy. Two groups of separators are investigated: PVC and glass mat.Two types of each group are analysed: PVC-R and PVC-E supplied by different manufacturers; and AGM and MAGM (modified AGM-new product developed by LABD at IEES). It has been established that: the PVC-R and PVC-E separators have similar porous structures; the AGM separator and MAGM separator have different pore size distribution, as clearly evidenced by the flow porometry data; though the glass mat separators have greater total pore volume (respective porosity), the PVC separators are characterized by greater permeability, because the pores in their narrowest part have greater diameters than those for the glass mat separators. The two methods used, mercury porosimetry and capillary flow porometry, give information about different characteristics of the porous structure. A combination of both methods will provide a more detailed information about the porous structure of the separators and a clearer idea about the dynamics of the processes that take place in the lead-acid batteries, than the data supplied by each of the techniques used alone. (c) 2005 Elsevier B.V. All rights reserved.