Rapid reversed-phase separation in the time-scale of seconds was conducted using methacrylate-based C18 monolithic capillary columns at high flow rates and elevated temperatures. The reversed-phase monolithic columns were prepared by in situ polymerization of octadecyl methacrylate (ODMA) and ethylene dimethacrylate (EDMA) within the confines of a polyimide-coated fused silica capillary. When a mixture of isoamyl alcohol and 1,4-butandiol was employed as a porogenic solvent, highly permeable monolithic stationary phases with adequate separation efficiency were reproducibly produced. The pressure drops of 20 cm-long columns were typically below 0.4 MPa at a normal flow rate of 3 mu L/min (linear velocity of I mm/s), and the theoretical plate numbers mostly reached 3000 plates/20 cm. These monolithic columns exhibited excellent mechanical strength as well as thermal stability, and they were properly operated even at a temperature of 80 degrees C and a high linear velocity of 100 mm/s. At 80 degrees C, the theoretical plate numbers exceeded 6000 plates/20 cm due to the enhanced mass transfer. Consequently, a series of alkylbenzenes, from toluene to amylbenzene, could be efficiently separated within 30 s on the 20 cm-long column by carrying out a ultra-high flow rate gradient at 80 degrees C.