The reaction of oxytetrahalides WOCl4 and WOBr4 with various unidentate and bidentate ligands containing Group V and Group VI donor atoms has been studied. Alkyl cyanides, pyridine, and cyclic ethers yielded 1 : 1 adducts, but bipyridyl, pyridine (excess), triphenylphosphine, triphenylarsine, and dialkyl sulphides all appeared to bring about reduction. Results of absorption spectra, magnetic susceptibility, and conductivity measurements are discussed.