Layer double hydroxides (LDHs) based on the hydrotalcite structure (Mg6Al2(OH)(16)CO(3)(.)4H(2)O) have been synthesized by coprecipitation, sol-gel and urea hydrolysis methods and with Mg:Al ratios of 2:1 and 5:1. Scanning electron microscopy shows the coprecipitated phases present the smallest individual crystallite sizes (ca. 150 nm) with the largest crystallites (2-4 mu m) for urea hydrolysis. Sol-gel samples show crystallites (150450 nm) fused together into much larger particles. The samples have been calcined at 723 K in flowing air to produce metal oxide phases which have then been rehydrated in the presence of hydroxide ions to produce meixnerite-like LDH phases (Mg6Al2(OH)(20)(.)4H(2)O). The base catalytic activity of these rehydrated samples has been measured by GC for the aldol self-condensation of acetone. Activity data has been correlated with sample characteristics to gain insight into the active sites and mode of action of these catalysts. (c) 2006 Elsevier B.V. All rights reserved.