Languages generally use musical pitch variation of the voice as part of their sound systems (Maddieson, 2011)--pitch variations that can be somewhat reminiscent of music. Music ability and/or training may influence language processing (e.g., Bidelman et al., 2011; Delogue et al., 2010). In particular, studies have concluded that there may be a relationship between absolute pitch and tone language acquisition (e.g., Lee, Lee, and Shi, 2011; Tillmann et al., 2011; Pfordresher and Brown, 2009). Other research has shown that fundamental frequency (F0) and F0 slope are crucial elements upon which native tone language speakers rely in tone perception (Guion and Pederson, 2007). With the given observations in mind, we could infer that an important tool in tone language processing and/or acquisition would be the ability to identify the relationship between notes (relative pitch ability). This study endeavors to explore the possible relationship between relative pitch aptitude or ability and adult L2 lexical tone perception/tone language comprehension. This study tested native Thai, Mandarin Chinese, and English-only speakers to determine each group's relative pitch proficiency and the possible correlation between this proficiency and lexical tone perception and tone language comprehension. The results of this study reveal that tone language speakers are more proficient at relative pitch perception. In addition, Thai speakers are more adept at perceiving the four lexical tones in Mandarin Chinese as well as perceiving Mandarin Chinese words than English-only speakers. Finally, this study found a moderate positive correlation between relative pitch proficiency and lexical tone perception for the English-only speakers but not for the Thai speakers. These findings lead to the conclusion that relative pitch proficiency may be relevant to non-tone language speakers endeavoring to learn a tone language. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]