The Guerbet reaction of ethanol to heavier products was performed over a series of extensively characterized carbonate-containing hydroxyapatites (HAPs) with different Ca/P ratios, and thus different densities, strengths and natures of acid and basic sites. These properties were correlated with the reactivity of the solids and an optimal ratio between the amount of acid and basic sites was evidenced (ca. 5). The best performances were accordingly obtained over a Hap-CO3 catalyst, which gave a yield of 30% in heavier alcohols at a 40% ethanol conversion.