Previous research [J. Pind, Acta Psychol. 89, 53–81 (1995)] has shown that preaspiration in Icelandic, an [h]-like sound inserted between a vowel and the following closure, can be cued by Voice Offset Time (VOffT), a speech cue which is the mirror image of Voice Onset Time (VOT). Research has also revealed that VOffT is much more sensitive to the duration of the neighboring vowel than is VOT [J. Pind, Q. J. Exp. Psychol. 49A, 745–764 (1996)]. This paper explores the hypothesis that it is primarily the perceived quantity of the vowel that is responsible for the effect of the vowel on the perception of preaspiration. This hypothesis is based on the linguistic fact that preaspiration can only follow a phonemically short vowel. This linguistic hypothesis is contrasted with an auditory hypothesis in terms of forward masking. Perceptual experiments show that the perceptual boundaries for preaspiration can be affected either by changing the preceding vowel’s duration or its spectrum. If the spectrum of the vowel changes towards that of a long vowel, longer VOffT’s are needed for listeners to perceive preaspiration, thus lending support to the linguistic hypothesis. © 1998 Acoustical Society of America.