This study investigates the phonetic realizations of Thai tones in monomoraic syllables produced with three different speeds. The results do not support previous claim that tones in monomoraic syllables are neutralized to mid, and lend support to a phonological representation in which the mora is associated to either H or no tone at all. In normal speech, a /H/ vs. /Ø/ opposition is found. In slow speech, a /H/ vs. /L/ distinction is found. Past research used the contrastive tonal status observed in slow speech as evidence to posit the underlying high and low tones for monomoraic syllables. In the present study, it is argued that the moraic association of the phonological low tone takes place as a result of glottal stop insertion.