Site-selective heterodimer formation in anthracene crystals containing a small amount of 2-hydroxyanthracene (2OHA) has been investigated. While direct irradiation at low temperatures of the absorption origins of the two 2OHA sites (O8 and O9) in anthracene did not lead to dimer formation, one of these sites (O9) is photoactive when irradiated at room temperature. From an analysis of the O8 and O9 fluorescence excitation spectra we infer an involvement of the X-traps in heterodimer formation. Furthermore it seems likely that an intermediate, perhaps an excimer is involved in the process. © 1978.