Lignocellulosic sugars are a potential feedstock for fuel and chemical production. The dehydration activity of an activated carbon catalyst was compared to the dehydration activity of homogeneous autocatalyzed xylose, glucose, and fructose dehydration using water as a solvent. Carbon catalysts are highly suitable for dehydration due to their water tolerance. Although furfurals were formed without any added catalyst, the presence of a heterogeneous catalyst affected beneficially the sugar isomerization and acid formation taking place in the same reaction conditions as sugar dehydration. Heterogeneous carbon catalyst guides the reaction towards the dehydration route by suppressing the formation of acids which cause the less selective autocatalytic reaction.