Perceptual grouping of the frequency components from a source into a single auditory object is needed when localizing a complex sound in an environment where other sounds are also present. Two acoustic regularities that might allow for such grouping are a harmonic relation among the components and a commonality of their spatial positions. The utility of these cues was examined in a forced choice psychophysical task by measuring sensitivity to interaural differences of time (IDT) for low-frequency stimuli presented via earphones. In the first experiment, stimuli were composed of either one, two, or three frequencies. A signal detection analysis used to predict the effects of combining information across frequencies found summation to be optimal, regardless of the harmonicity of the complex. A second experiment presented two-frequency complexes in which one tone, the target, contained the IDT to be detected while the other, the distractor, was constant across all three intervals of the forced choice. For inharmonic complexes, performance for the target-distractor combinations was equivalent to that found for targets presented alone, suggesting segregation of the targets and distractors into separate auditory objects. However, for harmonic target-distractor combinations, performance was diminished. A signal detection analysis of these data supports the idea that for purposes of lateralization, the interaural information in the targets and distractors was combined into a variance-weighted value, even though it meant a lowering of performance. Thus it seems that for the grouping of complex acoustic stimuli in space, harmonic structure is more important than commonality of spatial position.