NEW MATERIAL:The compound of formula I [Z is group of formula II (R1 is ≥3C alkyl; X is S or O) and T is -OR2 (R2 is 1W6C alkyl), or Z is group of formula III (R3 is ≥3C alkyl) and T is H; Y is acid residue which may form a double salt with a metallic salt].

EXAMPLE: 4-(4'-Octyloxyphenylthio)-2,5-diethoxybenzenediazonium hexafluoro phosphate.

USE: Useful as a photo-fixing heat-sensitiv recording medium. It has long alkyl chain and high thermal sensitivity and fixing sensitivity and gives low highlight density after fixing.

PREPARATION: The compound of formula I can be produced by diazotizing the compound of formula IV with sodium nitrite.