English as an International Language (EIL) is grounded in the concept of multiplicity. Such proliferation of non-native varieties of English leads to several controversies including the intelligibility of its speakers to listeners from various language backgrounds. Although this concern has been continuously addressed in EIL research, the focus was mainly on major ESL accents. English language educators form English periphery accents know very little about the scenario of their own English in relation to its use in international settings. This study explored the Thai-accented English pronunciation features that result in intelligibility failure. It employed the innovative and comprehensible intelligibility measurement of which the results can be applied to other English accents with the similar phonology patterns such as tonal and syllabic-timing languages as most of languages in Asian mainland continent. Thai-accented English spontaneous speech was measured for overall intelligibility using a transcription task performed by the listeners from a variety of different L1 backgrounds. The errors in transcription were phonetically analysed to ascertain which phonological features of Thai English pronunciation which led to a detriment in intelligibility. These features; hence, must be the focus in teaching English for international communicative purposes. Analysis of the results revealed that the salient features identified as posing the highest threat to international intelligibility were cluster simplification, consonant de-voicing, lack of final consonant released, and fully stressed unstressed vowels.