3-Methoxydiphenylamine-4-diazonium salt (MDDS) and its diazoresin were synthesized. The photo- and thermal decomposition of salt and the resins were investigated. The results show that MDDS exhibits excellent thermostability as well as high photosensitivity. A series of diazoresins with different organic counteranions, which dissolve in common solvents and usually were chosen to manufacture the negative presensitized plate, were also synthesized The kinetics of the photo- and thermal decomposition of MDDS in ethanol is reported.