The structural transformation of a MnO₂–CuO-containing catalyst supported on X-ray amorphous Al₂O₃ used in oxidative conversion of ethyl acetate in water under sub-critical (p = 200 bar, T = 633 K) and supercritical reaction conditions (p = 240 bar, T = 673 K) has been studied by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). TEM investigations revealed very small primary crystallites of the parent sample (ca. 5–20 nm), in contrast to a catalyst specimen exposed to supercritical water for ca. 100–200 hours which exhibited large micron-sized crystallites. This crystallisation process is accompanied by a drastic decline in the catalyst BET surface area. XRD studies support this observation: the basic material is nearly X-ray amorphous whereas the used catalyst shows several crystalline phases (e.g. Mn₂O₃, Al₂O₃, AlO(OH)). The patterns of spent catalysts taken from the reactor after different running times also gave evidence for the existence of mixed manganese copper oxides. Furthermore, XRD analysis of the used catalysts revealed a reduction of the basic MnO₂ to Mn₃O₄ and Mn₂O₃; this was also clearly proven by XPS analysis. Hydrolysis runs at supercritical water oxidation conditions showed that ethyl acetate is only hydrolysed into ethanol and acetic acid but, in contrast to these non-catalysed runs, the further oxidation of the hydrolysis products to carbon dioxide and water occurs in the presence of the oxidation catalyst. It may be stated that during time-on-stream up to ca. 200 h neither activity nor selectivity was found to significantly change, apart from the found structural alterations.