In order to determine if speech spectrograms can be used to identify human beings, two questions must be studied: (1) does the formant structure of phonemes uttered by a certain speaker change over a long interval of time, and (2) can the formant structure be changed by disguise, or is it even possible to imitate the formant structure of another speaker? Spectrograms of utterances produced by seven speakers and recorded over periods of up to 29 years showed that the frequency position of formants and pitch of voiced sounds shift to lower frequencies with increasing age of test persons. Speech spectrograms of texts spoken in a normal and a disguised voice revealed strong variations in formant structure. Speech spectrograms of utterances of well‐known people have been compared with those of imitators. The imitators succeeded in varying the formant structure and fundamental frequency of their voices, but they were not able to adapt these parameters to match or even be similar to those of imitated persons.