Abstract The decomposition reactions of neodymium phosphate, NdPO 4 , during mechanical milling (MM) have been studied. It has been found that under optimum conditions, NaOH, Ca(OH) 2 , CaO with CaCl 2 , and CaO with Ca(OH) 2 decomposed NdPO 4 completely, producing respectively Nd(OH) 3 with Na 3 PO 4 , Nd(OH) 3 with Ca 5 (PO 4 ) 3 (OH), NdOCl with Ca 5 (PO 4 ) 3 Cl and Nd 2 O 3 with Ca 5 (PO 4 ) 3 (OH). CaO by itself caused partial decomposition of NdPO 4 to Nd 2 O 3 , with the formation of calcium neodymium phosphate oxide, Ca 8 Nd 2 (PO 4 ) 6 O 2 . No chemical reactions occurred between NdPO 4 and CaCl 2 , NaCl or Na 2 CO 3 during milling. Powder X-ray diffractometry and transmission electron microscopy have been used to characterize the decomposition products. The Gibbs free-energy changes of all the reactions under study are calculated. It appears that the free-energy changes at room temperature (25 °C) and ambient pressure (1 atm) are crucial in determining whether reactions can occur during MM.