Although the first two formants are assumed to define vowel quality, the exact number of formants necessary for identification in the case of children and adults seem to be debated. The experiments reported here compare perceptual judgments on two- vs. five-formant vowels. Four- and six-year old children, and adults took part in the listening experiments. The results support the claim that the property detectors work in terms of vowel quality, according to the phoneme inventory concerned. The hypothesis about the different operations of the property detectors during and after speech acquisition has also been confirmed.