The influence of varying fundamental frequency on the perception of vowel quality in synthesized vowels was tested in two experiments. In experiment 1, based on investigations of natural Standard German vowel sounds, various model formant patterns F1’ to F3’ were created and, for each single pattern, sounds were synthesised on two or three fundamental frequencies (range 200-600 Hz). In experiment 2, corresponding to open-tube resonance characteristics for men, women and children respectively, sounds were synthesised with formant patterns F1’ to F5’, formant frequencies being odd multiples of 500, 600, or 700 Hz and fundamental frequencies being 1/3, 1/2 or 1/1 of the first formant frequency. Five phonetic expert listeners identified all synthesised sounds in a multiple-choice identification tasks. The results of both experiments revealed that the perceived vowel quality can be changed systematically by varying fundamental frequency only and that the changes can exceed the perceptual boundaries of two neighboring vowels. Further, sounds related to open-tube resonance patterns are not consistently perceived as neutral schwa vowels when fundamental frequency substantially varies. Thus, the result of both experiments strongly confirm previous claims of formant pattern ambiguity as well as of spectral envelope ambiguity of vowel sounds.