A method is described for estimating vowel formant frequencies from linear combinations of cepstral coefficients. The method is robust in that large errors from misidentified peaks are rare. It can be explained in terms of a Fourier cosine series of the desired identity between formant estimates and their true values. This analysis also sheds some light on the high correlations between the formants and dimensions of the log spectrum reported by Pols et al. [J. Acoust. Soc. Am. 53, 1093–1101 (1973)].