We have observed a pressure induced phase transition in KTbP 2 Se 6 at about 9.2GPa which was accompanied by a discontinuous jump of the absorption edge of about 0.5eV into the red. We have proposed that the high pressure phase is due to charge transfer from the selenium to the phosphorous accompanied by the formation of a Se–Se bond. In order to exclude a model which involves a 4f–5d charge transfer in Tb, leading to the intervalence charge transfer from Tb 3+ P 4+ –Tb 4+ P 3+ ,we have measured the pressure dependence of the luminescence of the crystal field split levels of the Tb-ion. A comparison of the pressure dependence of the luminescence spectra of KTb(MoO 4 ) 2 and KTbP 2 Se 6 shows that such a charge transfer does not occur in KTbP 2 Se 6 .