Even though Optimality Theory (OT) has come into prominence since the 1990s and it has been studied with many languages, surprisingly not much work has been done in the Thai language. So, exploration of how well this theory works, and where it fails, can lead to significant new insights into what happens when English loanwords are adapted to the Thai phonological system. This study pays close attention to the mechanisms required in understanding how conflict between faithfulness and markedness constraints is resolved through OT grammars, and in what ways. Not only is the major focus on the context-free adaptation of consonants that share direct correspondents between English and Thai, but it includes the adaptation that lacks direct correspondents between the two languages as well. Selected aspects of English loanwords cover the phenomena of consonants in different environments, namely, onset/coda simplification, laryngeal features, medial consonants, and liquid alternations. Both...