Although pre-low raising (PLR) has been extensively studied as a type of contextual tonal variation, its underlying mechanism is barely understood. This paper explored the effects of phonetic vs phonological duration on PLR in Cantonese and Thai and examined how speech rate and vowel quantity interact with its realization in these languages, respectively. The results for Cantonese revealed that PLR always occurred before a large falling excursion (i.e., high-low); in other tonal contexts, it was observed more often in faster speech. In the Thai corpus, PLR also occurred before large falling excursions, and there was more PLR in short vowels. These results are discussed in terms of possible accounts of the underlying mechanism of PLR. Copyright © 2021 Acoustical Society of America.