Stable alkali metal trialkylmagnesiate compositions are produced in high yields by simultaneously adding to a stirred dispersion of an alkali metal in an inert liquid hydrocarbon solvent a soluble dialkylmagnesium compound and an alkyl halide. By using a 2-alkyl substituted C.sub.4 -C.sub.18 primary alkyl chloride as the alkyl halide, alkali metal trialkylmagnesiates are produced which have excellent solubility, even at low temperatures, and good thermal stability over a wide range of temperatures. Moreover, the trialkylmagnesiates are soluble in liquid aliphatic or cycloaliphatic solvents in the absence of aromatic solvents or Lewis bases, which makes these compositions especially useful for certain applications, such as catalysts in anionic polymerization, where the presence of aromatic solvents or Lewis bases is undesirable.