The present invention provides heat-sensitive coating compositions, which comprise a color developer of formula (1) or mixtures thereof wherein R1 can be hydrogen, C1-20-alkyl, C3-8-cycloalkyl, C2-10-alkenyl, aryl or SO3H, and R2 and R3 can be the same or different and can be hydrogen, halogen, C1-20-alkyl, C3-8-cyclo-alkyl, C2-10-alkenyl, aryl, OR6, NR7R8, SR9, SO3H or COOR10 and R4 and R5 can be the same or different, and can be hydrogen, halogen, C1-20-alkyl, C3-8-cyclo-alkyl, C2-10-alkenyl, aryl, OR6, NR7R8 or SR9, R6, R7, R8, R9 and R10 can be the same or different and can be hydrogen, C1-30-alkyl, C3-8-cycloalkyl, C2-10-alkenyl or aryl, wherein C1-20-alkyl can be unsubstituted or substituted with one or more C3-8-cycloalkyl, C2-10-alkenyl, phenyl, halogen, OR11, NR12R13, SR14, SO3H or COOR15, and aryl can be unsubstituted or substituted with one or more halogen, C1-10-alkyl, halogenated C1-10-alkyl, C3-8-cycloalkyl C2-10-alkenyl, phenyl, OR11, NR12R13, SR14, SO3H or COOR15, wherein R11, R12, R13, R14 and R15 can be the same or different and can be hydrogen, C1-10-alkyl, C3-8-cycloalkyl or C2-10-alkenyl, a process for the preparation of these compositions, a process of coating substrates with these compositions, substrates coated with these compositions, a process for preparing marked substrates using these compositions, marked substrates obtainable by the latter process, and certain color developers.