The structure and morphology as well as the mechanism of formation of porous polyelectrolyte multilayers consisting of linear poly(ethylenimine) (LPEI) and poly(acrylic acid) (PAA) have been systematically investigated as a function of pH. The structures obtained exhibit dramatic differences with small changes in the pH of multilayer assembly and pH of postassembly treatment, yielding an observed range of pore sizes from tens of nanometers to micrometers and pore volume fractions from 0 to 77%. The porous phase transition is quite rapid (< 20 min), and structures observed include asymmetric membranes and isolated craters. It is thought that asymmetric membranes are achieved due to the high mobility of LPEI, which can exhibit interdiffusion when layered with PAA. To further understand the nature of the porous LbL multilayers, the pores were filled with liquid electrolyte and the impedance response of electrolyte-filled porous multilayers was examined; two time constants or two dry-state room temperature conductivities on the order of 10(-6) and 10(-9) S cm(-1) were observed. The asymmetric membrane LbL structure, first reported here, holds many potential applications in terms of filtration, catalysis, drug delivery, etc.