Polysaccharide solids including excess water were investigated by electrochemical impedance spectroscopy (EIS) with a three-electrode system. The ionic conductivities in the polysaccharide solids were studied as a function of pH of the aqueous phase or hardness of the solid. For the polysaccharides, agarose and K-carrageenan were used. Electrochemical impedance spectra were measured in the polysaccharide solid showing that both the ionic conductivity of the polysaccharide solid and the double-layer capacitance on the electrode surface are the same as those in an aqueous solution of the ions. Impedance spectra of 1-5 wt% agarose containing 0.1 M KCl showed almost linear relations at all frequencies but the spectra changed with the polysaccharide concentration. From 1 to 3 wt% the conductivity decreased with an increase in the agarose concentration, while from 3 to 5 wt% the conductivity increased with the increase in the hardness of the agarose solid. These results were interpreted by the growth of a three-dimensional network structure. The conductivity of the agarose solids containing 0.1 M KCl was large both in strong acidic and strong basic conditions, which was attributed to an increase in the H+ or OH-concentration. The conductivity of the solids was discussed in relation to their hardness. Thus, it was elucidated that the polysaccharide solid containing excess water can be used as a new ionically conductive solid for conventional electrochemistry. (C) 2004 Elsevier B.V. All rights reserved.