Isochrony has been considered only in terms of stressed syllables. However, it may also be a random property of unstressed syllables, and a control experiment was deemed necessary. A handtranscribed database of 98 sentences, each produced by three speakers, formed the input to an algorithm calculating durations of feet, number of syllables per foot, and mean syllable duration within each foot. In each output dataset, feet were based on one of the following criteria: stressed, tense, unreduced, random, or arbitrary syllables (the latter based on ordinal numbers of syllables within the utterance). Calculations were made of the correlations between foot duration and number of syllables per foot, and between mean syllable duration and number of syllables per foot. The 'foot compression effect' was shown to be nonrandom, and due to linguistic rather than arbitrary factors. A detailed examination of actual syllable durations was then carried out. The main determinants of syllable duration were the number of constituent segments, and syllable status in terms of target/nontarget. A small but significant syllable shortening effect was also found, dependent on the number of syllables in the foot, which was linguistic rather than random.