We show that in the aqueous-phase dehydration of fructose to 5-hydroxymethyl furfural (HMF) with zeolite catalysts, about 1-2 wt% of the zeolite dissolves, resulting in a solution of aluminosilicate species with Al 1.2-26.2 mg/L and Si up to 226 mg/L. The actual concentration of these species depends on the zeolite type used and on its Si/Al ratio. These species homogeneously catalyze side reactions of fructose and sequential reactions of HMF, but not glucose-fructose isomerization. This is in contrast to what has been observed using zeolites and several Al3+ salts. Fructose conversion to glucose, HMF, and levulinic acid was observed with several dissolved Al and Si salts (e.g., aluminum formate, Al(HCO2)(3), aluminum lactate, Al(C3H5O3)(3), and AlCl3). Detailed characterization using NMR, FTIR, DLS and XRD suggests that the active species dissolved from the zeolites are probably oligomeric aluminosilicate fragments in which the Al-O-Si and Si-O-Si connectivity is similar to that of amorphous aluminosilicate solids that do not catalyze hexose isomerization reactions. (C) 2013 Elsevier B.V. All rights reserved.