A thermal transfer printing sheet, suitable for use in a thermal transfer printing process, comprising a substrate having a coating comprising a dye of the formula: ##STR1## wherein X represents nitro or --COOR.sup.1 in which R.sup.1 is an optionally substituted hydrocarbyl radical;Y represents an optionally substituted C.sub.1-10 -alkyl or alkoxy radical;Z represents an alkyl radical, andR represents an alkyl radical which may be interrupted by one or two --O-- or --COO-- links.