Acidic aqueous solutions containing pyrrole and alkoxysulfonated PEDOT derivative (PEDOT-S) were found to undergo polymerization in the absence of an external oxidizing agent. The product was a nearly black-colored conducting hydrogel that after separation could be dispersed in water or acetone. The suspensions could be used to deposit cast films on a polycrystalline gold electrode. The polymer modified electrode showed a nearly Nernstian potentiometric response to Ag+ cations in the concentration range of 10−5–10−1 M with the slope of 54 mV/decade. The response was specific to Ag+ compared to a series of alkali and transition-metal cations (pKAg/M>3.7).