<p dir="ltr">Comprehensibility refers to the degree of effort that a listener requires to understand a speaker’s utterance (Derwing & Munro, 1997; Munro & Derwing, 1995; Derwing & Munro, 2005). Previous studies on L2 pronunciation have found that segmental errors could affect comprehensibility (Derwing et al., 1998; Derwing & Munro, 1997; Isaacs & Thomson, 2020; Saito et al., 2017). However, this finding only indicates a correlation between the number of segmental errors and lower comprehensibility. It is still unclear what aspects of pronunciation L2 learners need to practice in order to improve the comprehensibility of their speech. This thesis proposes that the degree of acoustic contrast may play a role in determining comprehensibility. More specifically, it investigates the relation between Mandarin speakers’ acoustic contrast between tense and lax vowels in English (i.e., [i] and [ɪ], [u] and [ʊ]) and the perceived comprehensibility of their speech.</p><p dir="ltr">A sentence production task and a comprehensibility rating task were conducted. In the sentence production task, 20 Mandarin speakers and 10 English speakers read aloud English sentences containing tense and lax vowels and were audio-recorded. The acoustic measurements of the vowels were taken in order to compare Mandarin speakers’ acoustic realization of the tense and lax vowels with English speakers’ productions. In the comprehensibility rating task, 48 English speakers transcribed the sentences recorded during the sentence production task and rated the comprehensibility of the sentences. The comprehensibility ratings were tested for correlation with the number of transcription errors, the degree of spectral and durational contrasts between vowels, and the speakers’ US residency length, in order to investigate the relation between intelligibility errors and comprehensibility, between acoustic contrast and comprehensibility, and between US residency length and comprehensibility.</p><p dir="ltr">The results of the linear-mixed effect model indicated that spectral contrast between [u] - [ʊ], but not [i] - [ɪ], was significantly reduced in Mandarin speakers’ productions compared to the English speakers, suggesting that Mandarin speakers under-differentiated the back vowel pair. A correlation test using Kendall’s <i>tau</i> indicated a significant negative correlation between number of intelligibility errors and comprehensibility, suggesting that intelligibility errors decreased comprehensibility. A correlation test using Kendall’s <i>tau</i> indicated a significant positive correlation between the degree of spectral contrast and comprehensibility rating for sentences with semantically meaningful context, suggesting that increasing the contrast between tense and lax vowels could help increase the comprehensibility of speech. Finally, a correlation test using Kendall’s <i>tau</i><i> </i>indicated no significant correlation between US residency length and comprehensibility, meaning that the relation between residency length and comprehensibility was not confirmed.</p><p dir="ltr">To sum up, acoustic contrast could be one of the contributors to speech comprehensibility. Therefore, directing English L2 learners to focus on increasing the acoustic distance between contrasting vowels could prove a fruitful strategy for improving the comprehensibility of L2 speech.</p>