In an inert atmosphere, the diamagnetic, mixed halide-methoxide clusters of the composition (M{sub 6}X{sub 12})(OCH{sub 3}){sub 2}{center dot}4CH{sub 3}OH (M = Nb, Ta; X = Cl, Br) are prepared by the exchange reaction of the terminal halides in the clusters (M{sub 6}X{sub 12})X{sub 2}{center dot}8H{sub 2}O by methoxide groups. The mixed halide-methoxide clusters are not soluble in methanol, but from suspensions in this solvent they dissolve slowly with further addition of alkali-metal methoxides. From concentrated alkaline solution in an oxygen-containing atmosphere a series of diamagnetic compounds M{prime}{sub 2}(Ta{sub 6}Cl{sub 12})(OCH{sub 3}){sub 6}{center dot}6CH{sub 3}OH (M{prime} = alkali metal) having all six terminal positions of the cluster units occupied by methoxide groups are isolated and their properties described.