Although pre-low raising (PLR) is an extensively studied contextual tonal variation, its underlying mechanism is barely understood. This paper explores the effect of duration on PLR in Cantonese and Thai, and examines how speech rate and vowel quantity interact with its realization in these languages respectively. Results for Cantonese revealed that PLR always occurred before a large falling excursion (i.e. T1T4); in other tonal contexts, it was observed more often in faster speech. In the Thai corpus, although PLR also occurred before large falling excursions, vowel quantity did not appear to influence its realization. These results are discussed in terms of possible accounts of the underlying mechanism of PLR.