Group 17 element fluorides EF(3) (E = Cl, Br, I) are well-known to undergo second-order Jahn-Teller symmetry breaking toward a T-shaped Ct, arrangement mainly due to a(1)’(HOMO)circle times e’(LUMO) mixing at the expected symmetric trigonal planar D-3h state. For heavy elements, the a(1)’HOMO is relativistically stabilized because of large element s-orbital participation. Hence, relativistic effects diminish the second-order Jahn-Teller term. This results in a large relativistic change in the F-eq-E-F-ax bonding angle of alpha(e)(R) - alpha(e)(NR) = 5.5 degrees in the case of AtF3 and causes an anomaly in the bond angle behavior down the group 17 compounds, alpha(ClF3) > alpha(BrF3) > alpha(AtF3) > alpha(IF3). Furthermore, the difference between the symmetric D-3h and the distorted C-2v structure Of AtF3 is only 10 kJ/mol at the coupled cluster level of theory, indicating that the measured F-eq-At-F-ax angle alpha(e) will be very sensitive upon the temperature applied in gas phase diffraction studies. Vibrational frequencies are predicted for all group 17 fluorides EF(3). As a consequence of the second-order Jahn-Teller distortion, the A(1) symmetric bending mode is strongly influenced by relativistic effects and becomes much lower in frequency compared to the B-1 out of plane mode for the heavier elements. With the exception of IF3, the symmetric D-3h structure represents a (metastable) weak local minimum at the MP2 level, rather than a transition state as expected. The D-3h point represents, however, a second-order saddle point at the HF level, and therefore, electron correlation seems to be responsible for changing the nature of the trigonal planar structure. Extended basis sets at the MP2 level as well as coupled cluster calculations were applied in order to obtain more accurate information for the energetics and structure of CIF3. These studies show, however, that the nature of the D-3h point is critically dependent upon the basis set (and the electron correlation procedure) applied.