A process for printing an image on a substrate comprising applying to the substrate an ink comprising a compound of Formula (1) or a salt thereof: ##STR00001## wherein: R.sup.1 and R.sup.2 are each independently H or a substituent; each X and Y independently is a substituent; and a and b are each independently 0, 1, 2, 3 or 4. The compounds used in the process have particularly good ozone fastness and are suitable for preparing inks which can be ink jet printed.