We report herein on the conversion of saturated and unsaturated fatty acids to alkanes over Pt/C in high-temperature water. The reactions were done with no added H2. The saturated fatty acids (stearic, palmitic, and lauric acid) gave the corresponding decarboxylation products (n-alkanes) with greater than 90 % selectivity, and the formation rates were independent of the fatty acid carbon number. The unsaturated fatty acids (oleic and linoleic acid) exhibited low selectivities to the decarboxylation product. Rather, the main pathway was hydrogenation to from stearic acid, the corresponding saturated fatty acid. This compound then underwent decarboxylation to form heptadecane. On the basis of these results, it appears that this reaction system promotes in situ H2 formation. This hydrothermal decarboxylation route represents a new path for using renewable resources to make molecules with value as liquid transportation fuels.