Speech is normally heard against a background of other sounds, yet our ability to isolate perceptually the speech of a particular talker is poorly understood. The experiments reported here illustrate two different ways in which a listener may decide whether a tone at a harmonic of a vowel’s fundamental forms part of the vowel. First, a tone that starts or stops at a different time from a vowel is less likely to be heard as part of that vowel than if it is simultaneous with it; moreover, this effect occurs regardless of whether the tone has been added to a normal vowel, or to a vowel that has already been reduced in energy at the tone’s frequency. Second, energy added simultaneously with a vowel, at a harmonic frequency near to the vowel’s first formant, may or may not be fully incorporated into the vowel percept, depending on its relation to the first formant: When the additional tone is just below the vowel’s first formant frequency, it is less likely to be incorporated than energy that is added at a frequency just above the first formant. Both experiments show that formants may only be estimated after properties of the sound wave have been grouped into different apparent sound sources. The first result illustrates a general auditory mechanism for performing perceptual grouping, while the second result illustrates a mechanism that may use a more specific constraint on vocal‐tract transfer functions.