A quartz capillary microreactor:is directly coupled to a mass spectrometer utilizing the exit of the reactor as a standard high-pressure free-jet molecular beam source. This direct sampling mass spectrometry (DSMS) design is used to-probe the reaction mechanism for the oxidation of acetic acid in supercritical water. The DSMS system is able to probe reactor conditions and to detect reaction intermediates, radicals, and product species during the early stages of reaction at the extreme pressures (> 23 MPa) and temperatures (400-500 OC)of interest to supercritical water oxidation; The oxidation of acetic acid with hydrogen peroxide in supercritical water was selected as a prototypical oxidation reaction to validate:our DSMS design. We report on our initial results, which include (1) the complete decomposition of the,hydrogen peroxide to oxygen before 450 degreesC, (2) the subsequent onset of reaction at 470 degreesC, (3) the identification and measurement of the HO2* radical, indicating hydrogen abstraction by oxygen as the dominate initiation step, and (4) the lack of any bulk water incorporation into the reaction mechanism up to 500 degreesC.