A method for preparing a high purity (e.g. greater than 70 wt. %) mono-hydrolyzed acyl halide compound as a precipitate from solution comprising the steps of preparing a solution comprising: i) at least 80 v/v % of a hydrocarbon solvent, ii) water at a molar concentration greater than its solubility limit within the solvent but less that its solubility limit in solution, iii) a tri-hydrocarbyl phosphate compound, and iv) a polyfunctional acyl halide compound at molar ratio to both water and the tri-hydrocarbyl phosphate compound of at least 1:1.