Psychophysical procedures were used to investigate the effects observed by humans when two odorants were presented simultaneously through the same nostril (physical mixture) or separate nostrils (dichorhinic mixture). The odor pairs were (+)-limonene and alpha-pinene, (+)-limonene and propionic acid, alpha-pinene and propionic acid. With both types of mixtures subjects indicated the acid had no effect on the perception of the other odorants. In contrast, limonene suppressed perception of the acid and pinene in physical and dichorhinic mixtures, pinene suppressed limonene in both types of mixtures but only suppressed the acid in physical mixtures. The magnitude of suppression with dichorhinic mixtures never exceeded that seen in physical mixtures, whilst suppression with physical mixtures was often significantly greater than in dichorhinic mixtures. The absence of suppression in dichorhinic mixtures in almost one-third of instances when suppression occurred with physical mixtures suggests that different mechanisms are involved in the perception of the two types of mixtures. A hypothesis that accounts for non-reciprocal suppression with both types of mixtures is proposed.