Disclosed is an electroconductive composition that has high electroconductivity and excellent heat resistance and is suitable for use as a solid electrolyte in a solid electrolytic capacitor. Also disclosed is a solid electrolytic capacitor that uses the electroconductive composition as a solid electrolyte and has low ESR and high reliability under high temperature conditions. A dispersion of theelectroconductive composition comprises an electroconductive polymer produced by oxidation polymerization of thiophene or a derivative of thiophene in water or an aqueous liquid formed of a mixture composed of water and a water-miscible solvent in the presence of polystyrenesulfonic acid and at least one material selected from phenolsulfonic acid novolak resins comprising repeating units represented by general formula (I) [wherein R represents hydrogen or a methyl group] and sulfonated polyesters; and a high-boiling solvent. The solid electrolytic capacitor comprises the electroconductive composition as a solid electrolyte.