Furfural promises to be a very important product of the lignocellulosic feedstock biorefinery. In this study the kinetics of both xylose and arabinose dehydration toward furfural is investigated in a dilute acidic medium under three different salt conditions. These comprise no salts, a 500 mM NaCl solution, and seawater. The results demonstrate that the salts catalyze all disappearance reactions of the pentoses, both toward furfural and toward loss products. Especially at higher temperatures, the increase in the reaction rate toward furfural is larger than toward loss products. The values of the reaction rate constants at different salt conditions and temperatures indicate that different ions catalyze specific (temperature dependent) reactions in the dehydration mechanism of a pentose. Furthermore, the increase in the reaction rates is more pronounced with the combined salts naturally present in seawater compared to only NaCl, even at the same salinity and ionic strength. It is shown in additional experiments with no acid added that the observed salt effects are independent of the acidic environment. Furthermore, the effects of the salts are larger for the dehydration of xylose compared to arabinose. Moreover, for all temperatures the molar furfural yield was improved by the addition of the salts. Finally, it is shown that the salts inhibit furfural loss reactions toward formic acid. The presented results further contribute to the understanding of the effects of saline catalysis on the mechanism of pentose dehydration.