Thermal imaging method

A method of thermal imaging which comprises heating imagewise a heat-sensitive element comprising a support carrying at least one imaging layer of a substantially colorless di- or triarylmethane imaging compound possessing within its triarylmethane structure an aryl group substituted in the ortho position to the meso carbon atom with a moiety ring-closed on the meso carbon atom to form a 5- or 6-membered ring, said moiety possessing a nitrogen atom bonded directly to said meso carbon atom and said nitrogen atom being bound to a group with a masked acyl substituent that undergoes fragmentation upon heating to liberate the acyl group for effecting intramolecular acylation of said nitrogen atom to form a new group in the ortho position that cannot bond to the meso carbon atom, said imagewise heating effecting the formation of said new group in the ortho position whereby said di- or triarylmethane compound is rendered colored in an imagewise pattern corresponding to said imagewise heating.