Increasing the concentration of an odorant increases the number of receptor cells and glomeruli in the olfactory bulb that are stimulated, and it is commonly acknowledged that these represent increased numbers of receptor types. Currently, it is not known whether a receptor type is associated with a unique quality and a unique molecular feature of an odorant, or its activation is used by the brain in a combinatorial manner with other activated receptor types to produce a characteristic quality. The present study investigated the proposal that a molecular feature common to several aliphatic odorants and known to be the key feature required to stimulate the same mitral cells in the olfactory bulb results in a quality that is common to the odorants. Since the common structural feature may activate a specific receptor type possibly at a similar concentration, the qualities of the odorants were determined at seven concentrations where the lowest and highest concentrations were the detection threshold (DT) and 729DT of each subject. A list of 146 descriptors was used by 15 subjects to describe the qualities of each odorant at each concentration. The results indicate that each of the five odorants was characterized by different qualities and the qualities of four of the odorants changed with changes in concentration. Importantly, no quality common to each of the odorants that had the same molecular feature could be identified and it is proposed that identification of the odorants occurs via a combinatorial mechanism involving several types of receptors.