An ink supply system for an ink jet printer, particularly a continuous ink jet printer, has a docking station where one of more disposable cartridges of ink or solvent are releasably connectable to the printer. The cartridge comprises an inner collapsible container with an outlet that is connectable to an inlet of the ink supply system and a substantially rigid outer housing. A front wall of the housing has an opening providing access to the outlet and a window that provides access to electrical contacts (92) associated with a memory device containing information relating to the contents of the cartridge. The contacts in a fixed relationship relative to the outlet so that when the cartridge is docked to establish fluid communication with the ink supply system the electrical contacts are electrically connected to respective contacts associated with the printer.