Quantitative study of the Photodecomposition of p -XC₆H₄CON₃ (X=H, F, Cl, Br, I) is reported. The heavy atom substitution did not change significantly the quantum yield of the photodecomposition. Quantum yield of the photodecomposition of p -iodobenzoylazide in an isopropanol solution was different from that in an cyclohexane solution, though the activation energy of the reaction in the both solutions were approximately same. The photodecomposition was considered to occur through the following reaction path: ArCON₃ (S_π-π^*)→S _{n -π*}→T _{n -π}^*T_π-π^*ArCON:(T) +N₂ (S), where, by the presence of the singlet and triplet n -π^* states between the two π-π * states, conversion of ArCON₃ (Sπ-π_*) to the Tπ-π^* state can occur very rapidly.