The invention provides a drop-on-demand ink jet printing apparatus for printing inks comprising water or a volatile solvent onto a continuous web substrate, the printing apparatus comprising: a print head, a chamber for retaining water or solvent vapor in the vicinity of the print head, the chamber having an inlet for the continuous web substrate and an outlet for the printed continuous web substrate, and means for feeding the continuous web substrate through the chamber inlet into the chamber, past the print head and out of the chamber outlet.