The ab initio molecular structures and vibrational frequencies for several transition metal heptoxides X2O7n− (n=0, 2, 4) were calculated using effective core potentials at the HF and DFT (B3LYP) levels. The relative merits of different valence basis set arrangements were tested by comparison with experimental results available, in particular with gas-phase Re2O7 molecular structure and vibrational frequencies. The calculations were then extended to other heptoxides of the VB, VIB and VIIB transition metal groups. The results indicate that a staggered geometry (either D3d or C2) is the energy minimum for most of the heptoxides studied. The only exceptions are Mn2O7, which clearly prefers an eclipsed C2v(syn) configuration, and Tc2O7, for which C2 and C2v(syn) geometries have nearly the same energy. Particular attention was given to the magnitude of the X–O–X bond angle, as this structural parameter has been a matter of some controversy. The calculated values range from 125° (Mn2O7) to 180° (Group VB heptoxides) and depend on the position of the transition metal in the Periodic Table. A tendency for linearity of the X–O–X moiety on going both downwards the group and backwards the period was observed and discussed.