A composition of matter comprising di-isoamylmagnesium and diethylmagnesium, with an isoamyl: ethyl alkyl group ratio of about 1.4 to about 4:1, which is soluble in aliphatic, cycloaliphatic, and aromatic hydrocarbon solvents is disclosed, as is a process for making such mixed dialkylmagnesium wherein magnesium metal is reacted with an ethyl halide and an isoamyl halide in the presence of the hydrocarbon solvent and in a substantially moisture-free and ether-free atmosphere, followed by separation of undissolved solids from the resulting solution.