A series of thorium(IV) perrhenato- and pertechnetato-complexes with PO donor ligands have been prepared and characterised both in the solid state and in solution. Isostructural complexes of general formula [Th(MO₄)₄(L)₄], where M = Re or Tc and L = triethylphosphate (TEP) (2 and 7), tri-iso-butylphosphate (TiBP) (3 and 8) and tri-n-butylphosphine oxide (TBPO) (4 and 9) have been prepared from the novel starting materials [Th(ReO₄)₄]·4H₂O (1) and [Th(TcO₄)₄]·4H₂O (6). The reaction of 1 or 6 with triphenylphosphine oxide (TPPO) in MeOH has also led to the synthesis of [Th(MO₄)₃(TPPO)₃(OCH₃)(HOCH₃)] (M = Re (5) or Tc (10)). While the structural characterisation of 4 and 9 has been previously described, we report for the first time the structural characterisation of 2 and 5, with a partial structural refinement of 3. Vibrational spectroscopic analysis confirms that the Tc complexes not characterised by single crystal X-ray diffraction are indeed isostructural with the perrhenate complexes with the same PO donor ligand. In all cases, monodentate coordination of the Group 7 tetraoxo anion is observed. ³¹P NMR spectroscopy indicates that in all the phosphine oxide-based complexes there is one dominant solution species. For the phosphate based systems, the presence of pertechnetate appears to inhibit PO donor ligand complexation in solution, whereas a significant proportion of each phosphate remains coordinated to Th^IV when perrhenate is present as the counter ligand. These results give some indication as to the mechanism of pertechnetate co-extraction with tetravalent cations in the presence of tri-n-butyl phosphate in the Plutonium and Uranium Recovery by Extraction (PUREX) process.