This paper highlights the importance of phonology in the acquisition of the English monophthong system and offers English-as-a-Second-Language (ESL) teachers practical suggestions for enhancing the efficacy of their daily classroom performance. The paper is based on a comparative analysis of phonetic mistakes made by Georgian and Turkish adult learners, who, apart from a few differences, share many characteristics. Analysis of these mistakes focuses on mistakes caused by sound similarity in English and the first language (L1) and mistakes caused by absence of the target sound in the L1. The paper explains that in acquiring the monophthong system of the English language, major obstacles for Georgian and Turkish learners are phonological. The main barrier is the phonological relevance of vowel length in English and stylistic relevance of the same feature in Georgian and Turkish. The Georgian and Turkish vowel systems are much simpler than that of English. This makes students single out certain reference sounds from their native sound reservoir in order to acquire the target vowels. The process results in the formation of reference sound group, which is an important source of mistake production. (SM)