This paper aims to account for the underlying causes of spelling errors made by Thai university students. A preponderance of Thai learners’ spelling errors is probably caused by the negative interference of the Thai writing system and the differences between the Thai and English writing systems, together with a dearth of knowledge of English phonological systems and poor lexical awareness. It seems clear that instruction in spelling has been neglected, and there is no systematic spelling instruction in English language teaching in Thailand. It is harder for readers to understand the words used, and the message that a writer is trying to communicate might not be transmitted as intended.