Using a food-rewarded two-choice instrumental conditioning paradigm the olfactory sensitivity of four adult female spider monkeys (Ateles geoffroyi) for eight aliphatic ketones was assessed. The results demonstrate a well-developed olfactory sensitivity of spider monkeys for aliphatic ketones. For the majority of odorants tested, all of the animals detected concentrations below 1 ppm. A significant negative correlation was found between the olfactory sensitivity of the spider monkeys and the number of carbons in the carbon chain of the homologous 2-ketones with different length. This indicates that carbon chain length systematically affects olfactory sensitivity in spider monkeys. No significant correlation was found between the positioning of the functional group on the C7-ketones and the olfactory sensitivity of the spider monkeys. In an across-species comparison between spider monkeys, squirrel monkeys, pigtail macaques and human subjects no systematic differences were found. This lends further support to the notion that the relative size of olfactory brain structures and the number of functional olfactory receptor genes do not correlate with olfactory sensitivity in primates.