To obtain high quality color images on plain papers using ink jet printing, the authors studied a new marking technology, named "double-component liquid system (DCLS)." The DCLS technology, which uses an ink set comprising pigment inks and treatment liquid, is a design concept that achieves high speed and high image quality by utilizing pigment agglomeration and vehicle penetration. The authors found that optical density and drying time can be described by the increase in viscosity of the ink-treatment liquid combination on admixture, which simulates sequential application of these components to the substrate. They compared various agglomerating agents (organic amine, multivalent metal salt, and organic acid) in treatment liquid by this measuring method and found that both high speed and high image quality could be achieved at the same time regardless of the types of agglomerating agents. Organic acids were found to demonstrate relatively high performance. The functional mechanism of organic acids can generally be explained by pH, and it was possible to control DCLS behavior by acid dissociation constant (pKa), neutralization of organic acid, and molar concentration in treatment liquid. DCLS ink set showed high-speed drying performance of 0.6 s or less (equivalent of 100 ppm or higher) and high image quality (optical density of 1.3 or more) at the same time on a variety of plain papers without using a heater.