The catalytic effect of an SUS316 reactor surface on the hydrolysis of benzamide in sub- and supercritical water was investigated at 350-450 degrees C and 25-65 MPa. Tube reactors having different surface area to volume ratios were used to distinguish the effect of the homogeneous reaction and the surface-catalyzed reaction. The surface of an SUS316 reactor acted as a heterogeneous catalyst for the hydrolysis reaction. Kinetic analysis indicated that surface Fe2O3 mainly contributes to the catalytic effect of SUS316. Both homogeneous and surface-catalyzed reactions occurred, and the contribution of each reaction depended on water density. The surface-catalyzed reaction was dominant at low water density, and the contribution from the homogeneous reaction became larger with increasing water density. Furthermore, at 400 degrees C and 25 MPa, hydrolysis products promoted the reaction.