Vocal duetting occurs when two individuals produce repeated, stereotyped vocalizations, often with alternating contributions. It evolved independently in many pair living taxa. Among hominoids, only hylobatids duet, but little is known about how mated pairs coordinate singing and if individuals adjust their song to spectral and temporal aspects of another’s song. If they do, this would demonstrate vocal flexibility not yet well documented in apes. To test this hypothesis, we analyzed duets of wild white-handed gibbons (Hylobates lar), quantifying female notes and the timing of male notes relative to them. We measured changes in female notes that preceded her great call phrase and that she initiated only after her mate had stopped singing. We predicted males would suspend their own song during a female’s great call phrase in anticipation of her climax, and thus should interrupt abnormal female calls, because these typically fail to climax. We compared (1) interrupted great call phrases females aborted, (2) interrupted phrases that were completed, and (3) uninterrupted, completed phrases. We found that abnormal great call phrases were interrupted by males, likely because the male anticipated that the phrase would be abandoned before reaching its climax. Although female call phrases varied in length, we also found that males replied in close synchrony with their ending. Subtle spectral and temporal variations in song influenced the timing of a mate’s singing and thus the structure and delivery of duets. Although ape vocal behavior is thought to be largely innate, our findings show unexpected flexibility in song expression.