Production of furfural by means of xylose degradation was investigated at 423 K with zeolites or an ion-exchange resin as a solid acid catalyst in water, water/toluene, or water/CO2. All the catalysts gave higher xylose conversions than did hydrothermal conditions without a catalyst. When the ion-exchange resin (Amberlyst 70) was used as the catalyst, furfural extraction with toluene effectively increased the yield relative to that of the reaction in water alone. The furfural yield obtained with the water/CO2 batch system was lower than that with the biphasic water/toluene system because the solubility of furfural in supercritical CO2 was lower than that in toluene. The furfural yield was increased by using a semibatch reactor system with continuous CO2 flow. The maximum furfural yield (52.3%) was achieved with Amberlyst 70 and a CO2 flow rate of 3.77 g min(-1) at 423 K.