Previous studies of vowel synthesis and of natural vowels have indicated: (1) a correlation between the lower formants and F0 for F0>175 Hz; (2) intelligibility and spectral differences of vowels with high F0>500 Hz; (3) severe methodological problems of formant frequency estimation for F0>300 Hz; (4) formant number alterations (appearance of different numbers of relevant formants for one vowel); (5) formant pattern ambiguity (appearance of the same formant pattern for different vowels). The purpose of this study was to replicate these findings. A large sample of the German vowels /i,y,e,(slashed oh),(cursive beta),(open aye),o,u/ produced in isolation as well as in CVC context was investigated. There were 18 674 recordings made of 35 men, 44 women, and 20 children. Isolated vowels were produced at different levels of F0, and with different intensities. A listening test was performed. Fourier spectra, LPC spectra and spectral constancy were analyzed. The results strongly support the five indications mentioned. In particular, for isolated vowel sounds the correlation between the formants and F0, the appearance of ``one-formant back vowels'' (only one apparent formant <2 kHz), and the formant pattern ambiguity for nonadjacent vowels proved to be very pronounced. Consequences for normalization approaches are discussed.