The kinetics of the acidulation reaction of phosphorite in nitric acid solutions was examined in order to elucidate the operating reaction mechanism and to delineate parameters of commercial significance to the nitrophosphate process. The reaction is positively influenced by stirring speed, temperature and concentration of nitric acid solutions. The experimental data fit the diffusion kinetic model and the overall reaction rate is transport controlled with an apparent activation energy of about 14 kJ/mol. (C) 2003 Published by Elsevier Science B.V.