This thesis is a continuation and expansion of Quam and Creel (2017a), who found that English-dominant native Mandarin speakers showed an attrition effect in their ability to process tones. Experiment 1 of this thesis aimed to replicate their findings, which were mostly upheld. Experiment 2 aimed to differentiate two alternative explanations for the selective tone attrition effects offered in Quam and Creel (2017a). One explanation is that lexical tones are more prone to attrition because of their inherent properties. A second explanation is that consistent use of English vowel categories reinforced the particular Mandarin vowel contrasts that were used, as opposed to the tones, which do not occur in English. We aimed to differentiate these explanations by testing Mandarin-English bilinguals on vowel contrasts that do not occur in English. Continued recruitment efforts will enable us to differentiate the two explanations.