Compounds Nb(OEt) 4X, Nb(OEt) 3X 2, Nb(OEt) 2X 3.CH 3COOEt and Nb(OEt)Cl 4.CH 3COOEt, NbOBr 3.CH 3COOEt (where X is chloride or bromide) have been isolated by the reactions of niobium penta-ethoxide with acetyl chloride and bromide. The reactions of niobium penta-isopropoxide and tert.-butoxide with acyl halides in equimolar ratios yield products of the type Nb(OR) 4X; the products with excess of acyl halides corresponds to NbOX 3.CH 3COOR. However, the reaction in the case of penta- tert.-butoxide and acetyl chloride yielded finally NbO(OAc) 0.75Cl 2.25 · CH 3COOBu t . The reaction of niobium penta-ethoxide with dry hydrogen chloride gas yielded triethoxide dichloride derivative. Only niobium tetra-alkoxide monochloride derivatives could be distilled or sublimed and these were found to be dimeric in boiling benzene.