The contact between Tai and Mon-Khmer (MK) languages has been dated back to pre-Proto-Tai . Sukhothai (C.13-16), the first attested Southwestern Tai language, also shows traces of the contact, especially with Old Mon (C.6-13) and Old Khmer (C.7-15). Some argue that they were in close contact to the extent that MK languages induced changes in Sukhothai. However, the contact situation has never been made clear.
Two proposals on contact situations between other medieval Thai languages and MK languages are (i) borrowing, as Thai speakers borrowed elements from MK and these elements induced changes  and (ii) interference through shift, as MK speakers shifting to speak Thai imported features from their source languages to Thai . In this paper, I argue that the contact situation between Sukhothai and the contemporary MK languages is borrowing, not interference through shift.
This paper investigates sound changes in Sukhothai from Proto-Southwestern Tai, its ancestor, if they are contact-induced changes and analyzes a plausible contact situation with MK languages. Three changes reported to occur in Sukhothai are studied: (i) the merger of *x and *kh, (ii) the merger of *aɯ and *aj, and (iii) the emergence of length distinction in closed-mid vowels.
I found that the only clear contact-induced change is the emergence of length contrast through loanwords. Other changes seem to be general tendencies of changes in the language family instead of contact-induced, as they are found in other Tai languages in Northern and Central Tai branches which are not in contact with MK. Results suggest that Sukhothai are in contact with contemporary MK languages, but the language contact situation is borrowing. Although there were MK speakers who shifted to speak Sukhothai, the number of shifting speakers might not be massive enough to play a major role in the language change. The proportion and types of MK loanwords also support this claim.
Based on the assumption that language contact situation reflects the relationship between populations, this paper provides insights into the population history in the Mainland Southeast Asia.
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