Structural and acid–base properties of Mg/Al and Mg/Ga layered double hydroxides (LDH's) obtained by microwave irradiation of the co-precipitated gels have been investigated and compared to those of samples conventionally aged by prolonged hydrothermal treatment of the gels. Similar crystallinities and chemical compositions were obtained whatever the synthesis method used. Besides, all samples, and remarkably the Ga-containing LDH's with a molar ratio Mg ∶ Ga = 4.5, exhibited pure lamellar phases. The acid–base properties of the mixed oxides obtained by calcination of the LDH's have been examined by microcalorimetric adsorption of CO₂ and by FTIR spectroscopy upon CH₃CN interaction. These techniques gave evidence that the number and strength of acid and basic sites were influenced by the nature and amount of the trivalent cation, as well as by the preparation method.