Previous studies in ink-jet printing have considered ink penetration into paper or ink evaporation to be effective in the drying process. In the present study, a unified approach that allows for simultaneous evaporation and penetration during the drying process was applied. To this end, penetration rates were measured experimentally using a Bristow tester. In addition, evaporation rates were determined from a theoretical evaporation model that contained no adjustable parameters. It was found that for plain paper used in the office environment, the rate of penetration was at least 20 × higher than the rate of evaporation. Accordingly, ink drying is determined mainly by penetration, and penetration curves alone are sufficient to predict the dry time or ink disappearance on a plain paper surface. In practice, this is illustrated by ink-jet print quality on plain papers of various sizing treatments. The sizing level needs to be adjusted according to the desired ink-jet image. That is, full-color printing requires penetration rates sufficient to accommodate each of the process inks applied without color bleed or other ink-to-ink interaction processes. This may be at expense of character and line print deterioration due to fiber swelling in mono-color printing.