The increasing knowledge of the energy level structure of the lanthanides has led to new applications for these ions. The Gd3+/Eu3+ quantum cutting couple was discovered a few years ago and may be applied in phosphors for fluorescent tubes based on a VUV xenon discharge. The VUV absorption of the phosphor LiGdF4:Eu3+ is poor and therefore a sensitizer is needed. This contribution describes an investigation of the use of the strong 4f12: →: 4f115d VUV absorption bands of thulium ions as a sensitizer for the Gd3+/Eu3+ quantum cutting couple. The incorporation of thulium in the quantum cutting phosphor LiGdF4 does result in sensitization of Gd3+ but not in downconversion in the Gd3+/Eu3+ couple. A competing downconversion process between gadolinium and thulium occurs resulting in infrared emission of thulium, which is not suitable for lighting applications.