A lexical gap exists in Thai, where certain consonants do not co-occur in syllables with certain tones. Phonological accounts have assumed that this lexical gap implies the presence of a phonological restriction among Thai speakers. However, there is very scant evidence in the form of (morpho-)phonological processes to support this claim. In order to determine whether these consonant-tone restrictions are present in the phonology of Thai speakers, a pair of perception experiments was done. A forced-choice judgment task between minimally different pairs of nonce words was run with the aid of 15 Thai-speaking participants. The results showed that Thai speakers do in fact have dispreferences for the consonant-tone combinations corresponding to the lexical gaps. While this result holds in native Thai nonce words, a second experiment that encouraged interpretation of stimuli as English loan words showed that Thai speakers do not apply these consonant-tone restrictions to English loans. These results are in accordance with previous observations from Japanese for example, where it is found that loan phonologies constitute a subset of a more stringent set of phonological constraints that hold in native phonologies. The results also show that Thai speakers have preferences that cannot be explained by the Thai grammar, but instead reflect the presence of cross-linguistic affinities between low tone and voiced stops.