The ability of supercritical water (SCW) to decompose heterocyclic compounds (quinoline and isoquinoline) has been explored in this study. The results obtained suggest that water acts as a chemical reagent above its critical point (374 degrees C and 22.1 MPa). Significant proportions of isoquinoline and quinoline were removed during the reaction with SCW. The response of these compounds to pyrolysis was also compared with their reaction with SCW. Both compounds were relatively more reactive in the presence of SCW than during pyrolysis. Because of the different positions of N atom in the two compounds, they reacted with SCW differently. Breaking of C-N bonds during SCW reaction was by hydrogenation and hydrocracking, while pyrolysis was due to thermocracking mainly.