The stability of ascorbic acid (AA) has been investigated under varying conditions, such as temperatures (80°C to 100°C), pressures (100 to 750 MPa), pH (5 and 7) and in presence of oxygen (0.25 mM). A biphasic model was selected to describe the time-dependent changes due to thermal treatment of AA in different buffer systems. Oxygen was found to be very important in affecting AA stability. Changes in temperatures from 80 to 100°C accelerate AA degradation, pH had different effect on the aerobic and anaerobic thermal degradation of AA and seemed not clearly effect on the degradation during high pressure thermal treatments. From the combined pressure-temperature experiments performed for longer time, AA was stable at pressures up to 700 MPa and temperatures up to 70°C after pressure build up and no further degradation of AA was observed up to 100 min of treatment.