The photochemical dehalogenation of 1,2,4-tribromobenzene, 1,2,3,5-tetrabromobenzene and pentachlorobenzene in open-air solutions of acetonitrile using natural and artificial sunlight as the irradiation source has been investigated. The regiochemistry of mono-dehalogenation has been determined for 1,2, 4-tribromobenzene and 1,2,3,5-tetrabromobenzene. Pentachlorobenzene-did not react. 1,2,4-Tribromobenzene yielded three products, 1,4-dibromobenzene, 1,3-dibromobenzene and 1,2-dibromobenzene with isomer percentages of 52 +/- 1%, 39 +/- 2% and 9 +/- 1%, respectively. 1,2,3,5-Tetrabromobenzene yielded 1,3,5-tribromobenzene, 1,2,4-tribromobenzene and 1,2,3-tribromobenzene with relative product percentages of 60 +/- 2%, 29 +/- 2%, 11 +/- 1%.