The present invention provides a highly conductive, highly voltage-resistant, and stable liquid electrolyte solution for capacitors which does not coagulate and is free from precipitation of salts in a wide temperature range, particularly at low temperatures, shows excellent electrical characteristics, and has excellent long-term reliability. The present invention also provides an electric double-layer capacitor and a lithium ion capacitor produced using the electrolyte solution for capacitors. The present invention relates to an electrolyte solution for capacitors including: an organic solvent; and a quaternary ammonium salt or lithium salt dissolved in the organic solvent, the organic solvent containing acetonitrile and a chain alkyl sulfonic compound represented by the formula (1): wherein R1 and R2, which may be the same as or different from each other, each independently represent a straight or branched chain C1-C4 alkyl group.