Sensitivity to frequency ratios is essential for the perceptual processing of complex sounds and the appreciation of music. This study assessed the effect of ratio simplicity on ratio discrimination for pure tones presented either simultaneously or sequentially. Each stimulus consisted of four 100-ms pure tones, equally spaced in terms of frequency ratio and presented at a low intensity to limit interactions in the auditory periphery. Listeners had to discriminate between a reference frequency ratio of 0.97 octave (about 1.96:1) and target frequency ratios, which were larger than the reference. In the simultaneous condition, the obtained psychometric functions were nonmonotonic: as the target frequency ratio increased from 0.98 octave to 1.04 octaves, discrimination performance initially increased, then decreased, and then increased again; performance was better when the target was exactly one octave (2:1) than when the target was slightly larger. In the sequential condition, by contrast, the psychometric functions were monotonic and there was no effect of frequency ratio simplicity. A control experiment verified that the nonmonotonicity observed in the simultaneous condition did not originate from peripheral interactions between the tones. Our results indicate that simultaneous octaves are recognized as "special" frequency intervals by a mechanism that is insensitive to the sign (positive or negative) of deviations from the octave, whereas this is apparently not the case for sequential octaves. (Contains 1 table, 5 figures, and 6 footnotes.)