To provide a reversibly thermochromic aqueous ink composition excellent in dispersion stability of a reversibly thermochromic microcapsule pigment and a writing instrument using the same.
Disclosed is a reversibly thermochromic aqueous ink composition comprising a reversibly thermochromic microcapsule pigment encapsulating a reversibly thermochromic composition comprising (a) an electron-donating color-developing organic compound, (b) an electron-accepting compound, and (c) a reaction medium to determine an occurrence temperature of a coloring reaction of the component (a) and the component (b), and a vehicle comprising water and a specific gravity adjuster selected from the group consisting of an oxyacid belonging to Group 6 having an atomic weight of 90 to 185 or salt thereof.