The frequency difference limen for steady-state formants was estimated many years ago [J. L. Flanagan, J. Acoust. Soc. Am. 27, 613--617 (1955)] and more recently [D. Kewley-Port and C. S. Watson, J. Acoust. Soc. Am. 95, 485--496 (1994)]. It was not generally realized, however, that the perceptual tolerance of transitions is somewhat greater [W. A. Ainsworth, Proc. 13th Int. Congr. Phon. Sci., Stockholm, pp. 837--840 (1995)] at least for vowel--vowel pairs. There appears to be a perceptual continuum from simple sounds (tone glides) to more complex, speechlike sounds [A. van Wieringen and L. Pols, J. Acoust. Soc. Am. 98, 1304--1312 (1995)]. In the present series of experiments the perceptual tolerance of the center of F1 and F2 vowel--vowel transitions was estimated. It was found that this tolerance is much reduced if the transition is not completed. This tolerance is also reduced if the center transition frequencies of both F1 and F2 were varied simultaneously.