An ink composition according to the present invention contains a polymerizable compound and a photopolymerization initiator, wherein an amount of organic sulfonic acid measured at a temperature of 25° C. by using a water extraction method is 50 ppm or less, and an amount of water measured by a Karl Fischer method is 0.50 mass % or less with respect to a total mass of the ink composition. Also, an ink-jet ink set according to the present invention includes the above-described ink composition of the present invention. Also, an ink-jet printing system according to the present invention uses the above-described ink composition of the present invention and an ink-jet recording apparatus, and the ink-jet recording apparatus includes an ink heating portion and an ink filter.