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The aldol condensation reactions of furfural/hydroxymethylfurfural (furfurals) with acetone/propanal in water–methanol solvents were studied over the solid base catalysts MgO–ZrO₂, NaY and nitrogen-substituted NaY (Nit-NaY). The reactions were conducted at 120°C and 750psig of He in batch reactors. Nit-NaY exhibited catalytic activity for aldol condensation comparable to MgO–ZrO₂ and much higher than that of NaY, indicative of the increased base strength after replacing the bridging oxygen with the lower electronegativity nitrogen over Nit-NaY. The aldol condensation of furfurals with acetone produces two different products, the monomer and the dimer. The monomer is formed from reaction of furfurals with acetone. The dimer is formed from reaction of the monomer with furfurals. MgO–ZrO₂ had a higher selectivity towards dimer formation. In contrast, Nit-NaY was more selective towards the monomer …