Three novel diimine Schiff bases including two asymmetric imines (2-OH)R-CH=N-C6H4-CH=N-R'(2-OH) type [where R=R'= phenyl for H2L1: R=naphthyl, R'=phenyl for H2L2 and R=R'= naphthyl for H2L3] have been synthesized with a new two step method. For this purpose, the starting Schiff bases 4-nitrobenzylidene-2-hydroxyaniline (SB1-NO2) and 4-nitrobenzylidene-2-hydroxy-3-naphthylamine (SB2-NO2) have been synthesized, previously. Nitro groups of them have been reduced into their amino derivatives (SB1-NH2 and SB2-NH2) with sodium dithionite as selective reductant and the other imino groups have been formed by adding salicylaldehyde or 2-hydroxy-1-naphthaldehyde to the same solutions. The structures of the diimine Schiff bases were confirmed by elemental analyses. ESI-MS, FT-IR, H-1 NMR and C-13 NMR spectroscopy. The phenol-imine and keto-amine tautomerism of the Schiff bases were investigated by FT-IR, 1 H NMR,13C NMR techniques and UV-vis spectra in different solvents (DMSO, methanol, chloroform, toluene and cyclohexane). The effects of acidic and basic media on the tautomeric equilibria were discussed. (c) 2010 Elsevier ay. All rights reserved.