A carbon-based monolithic acid catalyst with a honeycomb structure (CMHC) was synthesized via carbonization of a resorcinol-formaldehyde resin and subsequent sulfonation using sulfuric acid. The CMHC has nearly straight flow-through macropores (e.g., 47-78 mu m) formed by the directional freezing of the resin precursor. These macropores are surrounded by thin carbon walls of approximately 6-mu m thickness that are micro- and mesoporous as characterized by nitrogen adsorption experiments. The straight macropores enable a low hydraulic resistance to a liquid flow as demonstrated by >1000-fold less pressure drops of water flowing through the CMHC than those calculated for a packed bed of spherical particles with the same diameter as the approximate wall thickness of the CMHC. The CMHC shows a stable catalytic activity in the liquid-phase esterification of acetic acid with ethanol at 333 K in a flow reaction system for 50 h of operation. These results show prospective features of the CMHC for applications in flow reaction systems.