In the FBX dosimeter solution, the excitation of xylenol orange (XO) produces maximum emission at 550–575 nm both at room and liquid nitrogen temperatures (about 85%) having a lifetime of 0.20–0.36 ns. In addition, at room temperature there is an emission at 350 nm for the excitation at 260 nm (about 15%) having a longer lifetime of 3.71– 4.01 ns. Benzoic acid (BA) has excitation at 284–295 nm and emission at 320–365 nm having a lifetime of 1.38 ns. In an aqueous solution containing 5 x 10-3 mol dm-3 BA, 2 x 10-4 mol dm-3 XO and 0.04 mol dm-3 H2SO4 there is no XO emission at 550 nm due to UV absorption at 260 nm by BA .In this solution, 2 emissions are observed near 350–360 nm, having lifetimes of 1.25 ns (89%) and 2.86 ns (11%). The wavelengths for the emission of XO and absorption of ferricXO complex are nearly the same. Excited XO produces oxidation of ferrous ions and BA increases the chain length.