The paper presents a case study of two popular US American CEOs. It compares the acoustic vowel space sizes of the more charismatic speaker Steve Jobs and those of the less charismatic speaker Mark Zuckerberg, as part of an initial acoustic step to examine a traditional claim of rhetoric that clearer speech makes a speaker sound more charismatic. Analysing about 2,000 long and short vowel tokens from representative keynote speech excerpts of the two speakers shows that Jobs' vowel space is, across various segmental and prosodic context factors, significantly larger than that of Zuckerberg, whose vowel space is strongly reduced particularly when addressing investors. The differences in vowel-space size are consistent with the claim of rhetoric that a clear articulation is a key characteristic of a charismatic speaker. The discussion of the results describes further experimental steps required to back up the link between clear pronunciation and speaker charisma.