The solution-mediated transformation of anhydrous L-phenylalanine in a saturated solution of monohydrate L-phenylalanine has been studied and the transformation rate quantified using the powder X-ray diffraction technique. It has been demonstrated that the anhydrous form is not stable below 37 degreesC (transition point) in the presence of additives, as previously reported (Sato, T.; Sano, C. European Patent 0703214A2, 1995), and that the additives (ammonium sulfate and dextrose) affect only the transition rate but not the thermodynamic stability or the transition point.