Abstract Among the key acoustic features attributed with the intelligibility gain of Clear speech are the observed reduction in speaking rate and expansion of vowel space, representing greater articulation and vowel discrimination. Considering the slower speaking rate, previous works have attempted to assess the intelligibility impact of time-scaling casual speech to mimic Clear speech. In a complementary fashion, this work addresses the latter of the key traits observed in Clear speech, notably vowel space expansion. Specifically, a novel Clear speechinspired frequency warping method is described and shown to successfully achieve vowel space expansion when applied to casual speech. The intelligibility impact resulting from this expansion is then evaluated objectively and subjectively through formal listening tests. Much like the relevant time-scaling works, the frequency warping that expands vowel space is not shown to yield intelligibility gains. The implications are thus that further analyses and studies are merited in order to isolate the pertinent acoustic-phonetic cues that lead to the improved intelligibility of Clear speech.