The effect of temperature and pressure on the batch hydrolysis of used cooking oil in fatty acids- (autocatalytic system), dilute acid- (pH 5.0), and dilute Cu-catalyzed systems (500 mg CuSO4 per kg solution) was studied. The reaction was carried out using a fixed water/acylglycerides molar ratio of 30 : 1 under various temperatures (200–250 °C) and initial pressures (1.96–4.90 MPa) for 2 h. Experimental data were processed statistically using ANOVA and the Spearman correlation method aside of conventional data interpretation. The Cu-catalyzed system was superior to the other systems. Temperature had greater effect on oil hydrolysis than initial pressure. The use of CO2 as the pressurizing gas altered fatty acid production and Cu soap formation particularly by oil dilution that also accompanied by a slight acidification of the aqueous phase. The water ionization constant was the most influencing property for fatty acid production and Cu soap formation.