A process is described wherein after formation of first crop of Vilsmeier-Haack reagent by reacting Phosphorus Pentachloride with N,N-dimethylformamide to form a first crop of Vilsmeier reagent as insoluble crystals, a by-product of this reaction, the Phosphorus Oxy-Chloride, reacts with N,N-dimethylformamide to give a second crop of Vilsmeier reagent. This second crop of Vilsmeier reagent is soluble in DMF. This process makes it possible to double the yield of chlorinated substrate, such as sucrose-6-acetate or sucrose-6-benzoate, from the same quantity of Phosphorus Pentachloride.