The enormous temperature dependence of the (2)J(PP) coupling constants in S(PF2)(2) and Se(PF2)(2) has been explained by a theoretical investigation of their conformations and NMR coupling constants. In contrast, the coupling in O(PF2)(2) is almost invariant. Gas electron diffraction data for S(PF2)(2) have been reinterpreted. The results show that two conformers, with C-s and C-2v symmetry, exist for the S and Se compounds. The C-s and C-2v conformers have very different (2)J(PP) coupling constants (-12.6 and 395.2 Hz for S(PF2)(2) at B3LYP/aug-cc-pVQZ) and thermal interconversion of these conformers explains the experimental behavior.