Influence of HCl pre-treatment conditions (temperature: 26 or 110degreesC and period: 0.1, 0.5 or 1.0 h) and calcination temperature (300, 600 and 800degreesC) of Ga-Mg-hydrotalcite (Mg/Ga = 3.0) anionic clay on its catalytic activity in the benzylation (by benzyl chloride) or benzoylation (by benzoyl chloride) of toluene, p-xylene, mesitylene and anisole at 110degreesC has been investigated. The hydrotalcite before and after its HCl pre-treatment or calcination is characterized by measuring its surface area and crystalline phases. Its catalytic activity in the benzylation reactions is increased very markedly by the HCl pre-treatment, depending upon the severity of the pre-treatment. It showed no activity in the benzoylation of substituted benzenes (for 3.0 h) but, after its pre-treatment by HCl gas (at 110degreesC for 1.0 h), it showed high activity for the benzoylation reactions. Its catalytic activity in the benzylation of toluene is also increased very markedly after its calcination (at 300-800degreesC): the higher the calcination temperature, the higher is the catalytic activity. The induction period for the benzylation reactions is drastically reduced after the HCl pre-treatment or calcination of the hydrotalcite. The catalyst activation by the HCl pre-treatment is attributed to the formation of catalytically active mixed GaCl3 and MGCl(2) species dispersed on MgO and Ga2O3. The catalyst activation by the calcination at the lower temperature (300degreesC) is attributed mostly to the dehydration of the hydrotalcite, causing an increase in the surface area. However, that at the higher temperatures is attributed to both the dehydration and the decomposition of the hydrotalcite, forming metal oxides (at 600degreesC) or a mixture of MgO and MgGa2O4 (at 800degreesC), with a large increase in the surface area of the catalyst.