We previously reported the ability of strontium hydroxyapatite to catalyze the conversion of ethanol into 1-butanol with higher selectivity than that reported for calcium hydroxyapatite catalysts. In the present study, we investigated the catalytic conversions of ethanol over substituted hydroxyapatites, such as Srio(PO4)(6)(OH)(2), Ca-10(VO4)(6)(OH)(2), Sr-10(VO4)(6)(OH)(2), and Ca-10(PO4)(6)(OH)(2), and their solid solutions, such as Ca10-zSrz(PO4)(6)(OH)(2), Ca-10(PO4)(6-x)(VO4)(6-x)(OH)(2), and Sr-10(PO4)(6-x)(VO4)(6-x)(OH)(2), were investigated. The strontium phosphate hydroxyapatite [Sr-10(PO4)(6)(OH)(2): Sr-P] exhibited the highest 1-butanol selectivity among the tested catalysts in the region of the ethanol conversions between 1 and 24%. The reaction mechanism of 1-butanol formation over the Sr-P hydroxyapatite catalyst includes the dehydrogenation of ethanol into acetaldehyde, the aldol condensation of acetaldehyde into crotonaldehyde, and the hydrogenations of crotonaldehyde, 2-buten-1-ol, and/or butyraldehyde into 1-butanol. The Sr-P hydroxyapatite catalyst showed high selectivity into crotonaldehyde in the aldol condensation of acetaldehyde and inhibited the coking in the hydrogen transfer reaction of 2-buten-1-ol into 1-butanol, which might be reasons why the Sr-P hydroxyapatite catalyst showed the high 1-butanol selectivity in the catalytic conversion of ethanol. (C) 2011 Elsevier B.V. All rights reserved.