A refill unit for refilling an ink cartridge that forms part of an inkjet printer. The printer also having a printhead, control circuitry and an interface for releasably engaging the refill unit. The refill unit has a body containing a quantity of ink, an ink outlet, and a memory circuit for storing information relating to at least one characteristic of the ink. During use the memory circuit allows to the control circuit to interrogate it to verify that the ink in the refill unit is suitable for the printer.