Carboxylic acids were selectively reduced to aldehydes when their mixture with formic acid in the form of vapour had been passed over solid catalyst. The activities of 20 wt.% MO2/A catalysts, where M = Mn, Ce or Zr and A = Al2O3 have been studied in the mentioned reaction at the temperature range of 573-723 K. Hexanoic acid and its branched monomethyl and dimethyl isomers were used as substrates. For hexanoic acid reduction (HCOOH/C5H11COOH molar ratio 3) the following order of the maximum yield of aldehyde (in %) has been observed: Mn(58) > Ce(36) >> Zr(5) > support(1). Ketonization of an acid into undecan-6-one was the main side reaction in the upper range of reaction temperatures. Small amounts of hexan-1-ol and methyl hexanoate have also been detected among reaction products. An increase in the molar ratio of HCOOH/C5H11COOH diminished the ketone formation. The reactivity of branched x-methylpentanoic (x = 2, 3 and 4) acids in the reduction by formic acid depended strongly on the position of the substituent in the acid molecule and decreased in the following order: 4-methyl- 3-methyl-> 2-methyl-. 2,2-Dimethyl- and 3,3-dimethylbutanoic acids were reduced to alkanals with very low yields (6%) even at the highest temperatures. The same order of reactivity of isomeric hexanoic acids has been observed in the homo-ketonization of pure acids. The MnO2/A catalyst has had a constant activity and selectivity in the reduction of hexanoic acid by formic acid. In the presence of HCOOH, methyl hexanoate was transferred below 673 K into hexanal and hexan-1-of with 12 and 5% yields, respectively. Under the same conditions hexanal was reduced to hexan-1-ol with moderate yields (23%). Above 648 K, the ketonization of methyl hexanoate or hexanal took place and undecan-6-one was found to be the main product in both cases (94-96% at 723 K). It has been also shown that neither dihydrogen nor the products of the decomposition of formic acid can reduce hexanoic acid in the presence of the studied catalytic systems. The highest yields of hexanal (up to 3%) were reached at 673 K for the H-2/C5H11COOH molar ratio equals 3. An increase in this ratio up to 15 resulted in an increase in the yield of hexanal up to 7%. A mechanism of the formation of an aldehyde in the reaction between formic and carboxylic acid has been proposed. It is assumed that a mixed formic-carboxylic anhydride is formed with its subsequent decarboxylation to an aldehyde. On the basis of XRD measurements of freshly prepared manganese catalyst, it was concluded that its active phase contains beta-MnO2 which is almost quantitatively reduced to MnO in contact with either formic or bexanoic acid or their mixture. (C) 2007 Elsevier B.V. All rights reserved.