A colorless toluene solution of benzo[1,2,3-kl:4,5,6-k'l']dixanthene endoperoxide (BDXepo) 1 slowly changes to red BDX 2, by refluxing at 110 degrees C in toluene. However, in the presence of HCl, 2 was formed more rapidly from 1 even at room temperature. A novel dosimetry system was proposed using 1 and acid generating materials such as chlorinated alkanes. A chloroform solution of 1 ([1] = 1.0 x 10(-3) M) gave red color of 2 after irradiation with gamma ray at 73 Gy.