ALKYPHOSPHONOTHIOIC DIHALIDES ARE PREPARED IN IMPROVED YIELDS BY REACTING AN ALKYL ALUMINUM SESQUIHALIDE WITH A THIOPHOSPHORYL HALIDE IN THE ABSENCE OF A HYDROCARBON SOLVENT. THE REACTION NEED NOT BE CARRIED OUT WITH PURE THIOPHOSPHORYL HALIDE. INSTEAD, THE THIOPHOSPHORYL HALIDE CAN BE IN THE MIXTURE PRODUCED BY REACTING POSPHOROUS TRIHALIDE AND SULFUR WITH AN ALUMINUM TRIHALIDE CATALYST.