The hypervalent adducts of SiF4, trans-[SiF4(R3PO)2] (R = Me, Et or Ph), cis-[SiF4{R2P(O)CH2P(O)R2}] (R = Me or Ph), cis-[SiF4(pyNO)2] and trans-[SiF4(DMSO)2] have been prepared from SiF4 and the ligands in anhydrous CH2Cl2, and characterised by microanalysis, IR and VT multinuclear (1H, 19F, 31P) NMR spectroscopy. The NMR studies show extensive dissociation at ambient temperatures in non-coordinating solvents, but mixtures of cis and trans isomers of the monodentate ligand complexes were identified at low temperatures. Crystal structures are reported for trans-[SiF4(R3PO)2] (R = Me or Ph), and cis-[SiF4(pyNO)2]. The GeF4 analogues cis-[GeF4{R2P(O)(CH2)nP(O)R2}] (R = Me or Ph, n = 1; R = Ph, n = 2) were similarly characterised and the structures of cis-[GeF4{R2P(O)CH2P(O)R2}] (R = Me or Ph) determined. The reaction of R3AsO (R = Me or Ph) with SiF4 does not give simple adducts, but forms [R3AsOH]+ cations as fluorosilicate salts. SiF4 adducts of some ether ligands (including THF, 12-crown-4) were also characterised by 19F NMR spectroscopy in solution at low temperatures (190 K), but are fully dissociated at room temperature. Attempts to isolate, or even to identify, SiF4 adducts with phosphine or thioether ligands in solution at 190 K were unsuccessful, contrasting with the recent isolation and detailed characterisation of GeF4 analogues. The chemistry of SiF4 with these oxygen donor ligands, and with soft donors (P, As, S or Se), is compared and contrasted with those of GeF4, SnF4 and SiCl4. The key energy factors determining stability of these complexes are discussed.