Abstract: Stable diorganomagnesiums are prepared which are soluble in acyclic liquid hydrocarbons and represent novel and highly useful compositions. They are prepared, for instance, by forming a mixture of magnesium metal powder in an acyclic liquid hydrocarbon to which are added a C.sub.1 -C.sub.4 n-alkyl halide, such as n-butyl chloride, and a n-alkyl halide containing at least 6 carbon atoms, such as n-octyl chloride, and effecting reaction to convert them to their corresponding dialkylmagnesiums. Then, a secondary or tertiary organolithium compound, such as sec- or tert-butyllithium, is added, in the form of a solution in an acyclic liquid hydrocarbon, and the byproduct halide salts which were formed and are present in the reaction mixture are separated from the resulting stable solution of the diorganomagnesium complex by filtration or by equivalent separation procedures.