This dissertation aims at investigating the speech act of disagreement as it is carried out linguistically in an English-language asynchronous online forum between unacquainted Americans and Thais (TEFL and TESL). The analysis and interpretation of the data are based on both "emic" and "etic" perspectives, in the framework of the pragmatic theory of politeness, cultural dimensions, and rapport management. Collection of natural data from the participants' performances online showed that disagreement was expressed in two ways: softened and aggravated. To soften the illocutionary force of disagreement, the participants of the three groups enhanced their interlocutors' face by means of positive politeness strategies, and off-record strategies. The interactional participants lessened the imposition of their opinions on their interlocutors' face and rights by employing negative politeness strategies. To strengthen their disagreement, one USA participant and two TEFL participants attacked or threatened their interlocutors' face, sociality rights and obligations by three means: 1) bald-on-record strategy, 2) negative politeness strategies, and 3) off-record strategy: rhetorical questions. Variables of politeness can be subsumed under three general headings: face concern and rapport management, culturally-specific conventions in interaction, and contextual and situational factors. These variables are intricately related during the participants' dynamic co-construction of their disagreement with their co-participants. There could be evidence of Thai English as a variant of World Englishes. Thai English, as exemplified in the aggravated disagreements, is deeply impacted by the different social domains in which it is used; in particular, the nature of topics or issues being discussed. It is also affected by Thai notions of politeness and face; the implications or nuances of which may vary from those of the American culture. The dissertation concludes with pedagogical recommendations for adopting Rees-Miller's (1995) three phases of raising pragmatic-awareness as an EFL teaching approach. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]