We conducted terephthalic acid (TPA) synthesis from p-xylene in high-temperature liquid water (HTW) at 300 degrees C. The p-xylene concentration at the reaction condition was 0.2 mol L-1, which is the highest to date in research that achieved at least 80 mol % yields of TPA in HTW. Pure oxygen gas was the oxidant. Increasing the MnBr2 catalyst concentration increased the rate of TPA formation only slightly. In contrast, whether oxygen was fed in small, quick, discrete bursts, or fed continuously significantly affected the p-xylene conversion and the TPA selectivity. Adding oxygen in quick bursts and small increments led to high selectivities (>90 mol %) of TPA. Continuous addition of oxygen failed to do so. In addition to identifying the sensitivity of this synthesis to the oxygen feed method, these results also demonstrate the feasibility of HTW for TPA synthesis at higher concentrations, and hence high TPA production per unit reactor volume. (c) 2009 American Institute of Chemical Engineers AIChE J, 55: 710716, 2009