The present paper reports results for the purification of phosphoric acid by liquid-liquid extraction with methylisobutylketone. The simultaneous effect of some operating parameters, namely concentration of the feed acid, massic ratio of solvent to feed, temperature and number of theoretical stages, on the performances of the extraction step has been examined using a factorial design 2(4). The established models allowed us to forecast the experimental conditions required to obtain a given yield of P2O5 and/or a well-defined quality of the stripped acid. The optimal conditions have been determined by using a desirability function.