An ink refill unit 155 is adapted for use with an ink cartridge that has a negatively pressurized ink storage compartment 46 that supplies ink to a printhead assembly 22, and an interface for receiving the refill unit 155. The refill unit has a body 157 containing a quantity of ink and a docking portion with an ink outlet. During use, the docking portion establishes fluid communication between the body and the ink storage compartment through the ink outlet upon releasable engagement with the interface, such that ink is drawn from the body into the cartridge because of the negative pressure in the ink storage compartment.