First density functional calculations on ReO 42Ϫ molecules embedded in KCl are reported. The results on the isolated ReO 4 2Ϫ molecule confirm the existence of a strong Jahn-Teller effect induced by the e vibrational mode and favoring a squashed D 2d tetrahedron as stable geometry, where the unpaired electron is placed in an ϳ3z 2 -r 2 orbital. Calculations carried out on an 81 atom cluster reveal that the Jahn-Teller distortion in KCl is essentially the same as that for the free ReO 4 2Ϫ molecule, which is thus in agreement with the geometry inferred from EPR data on KCl: ReO 4 2Ϫ . Despite this fact, the Cl Ϫ 3 ReO 4 2Ϫ replacement is shown to produce outwards or inwards relaxations on close ions in the KCl lattice, whose origin is briefly discussed.