Reactions of acetic acid were conducted with alkali (KOH) and zirconia (ZrO2) catalysts in supercritical water (SCW) (673 K and 25 MPa-40 MPa). For comparison, we also conducted the experiments without catalyst and/or water at the same reaction conditions. Decarboxylation of acetic acid with zirconia in supercritical water formed acetone, CO2, and water without forming any other compound. On the other hand, decomposition of acetic acid with alkali produced methane (CH4) and CO2 with acetone. Without water and/or catalyst, the main products were also CO2 and CH4 (without catalyst) or acetone (with ZrO2); however, by-products, such as a water-insoluble compound were formed. The structure of zirconia was changed during reaction in supercritical water. The rate of decarboxylation of acetic acid exerted an effect on the particle size of zirconia.