A process for printing an image on a substrate comprising applying to the substrate an ink comprising a medium and a mono azo compound of Formula (1) or a salt thereof: ##STR00001## wherein: A and X are independently optionally substituted aryl or heteroaryl; B is independently optionally substituted arylene or heteroarylene, in either case comprising a phenylene ring which is attached to the --N.dbd.N-- and --NR.sup.a-- groups in the compound of Formula (1) in the para positions; R.sup.a and R.sup.b are independently H, optionally substituted alkyl or optionally substituted aryl; the compound of Formula (1) or a salt thereof is a water-soluble dye; andthe ink is applied to the substrate by means of an ink jet printer.