The optical quenching spectra in CdS1−xSex (0 ≦ x ≦ 1) crystals of high photosensitivity were investigated as a function of temperature using polarized light. The results suggest that the quenching spectrum in CdS1−xSex is made up of a high energy band which is asymmetric and a low energy band which is of Pekarian shape. The energy of phonons which cause thermal broadening of the low energy quenching band in CdS1−xSex is in the order of the energy of LA phonons in those materials and decreases monotonically with increasing x. It is also found that the ground and the excited level of the sensitizing centres (assumed to be cadmium vacancies) in CdS1−xSex monotonically shift towards the valence band with increasing x. This shift is attributed to a replacement of S by Se in the surrounding of the cadmium vacancy.