Subjects estimated the odor intensity of mixtures of two odorants, 1-propanol and n-amyl butyrate. Sometimes the two substances were mixed in the vapor phase (physical mixtures), and sometimes they were presented separately to the two nostrils simultaneously (dichorhinic mixtures). The two types of mixtures yielded similar results. In all instances, the perceived intensity of the mixtures was lower than the sum of the intensities of the unmixed components, and in some instances, the intensity of the mixtures even fell between the intensities of the unmixed components. Each component of the mixtures displayed some ability to mask the other component. A model of vector summation provided a good description of the relation between the intensity of a mixture and the intensities of the unmixed components. The experiments reveal the general nature of how chemically unreactive deodorizers and masking agents exert their effects.