This article reports an acoustic study analysing the time-varying spectral properties of word-initial English liquids produced by 31 first-language (L1) Japanese and 14 L1 English speakers. While it is widely accepted that L1 Japanese speakers have difficulty in producing English /l/ and /ɹ/, the temporal characteristics of L2 English liquids are not well-understood, even in light of previous findings that English liquids show dynamic properties. In this study, the distance between the first and second formants (F2–F1) and the third formant (F3) are analysed dynamically over liquid-vowel intervals in three vowel contexts using generalised additive mixed models (GAMMs). The results demonstrate that L1 Japanese speakers produce word-initial English liquids with stronger vocalic coarticulation than L1 English speakers. L1 Japanese speakers may have difficulty in dissociating F2–F1 between the liquid and the vowel to a varying degree, depending on the vowel context, which could be related to perceptual factors. This article shows that dynamic information uncovers specific challenges that L1 Japanese speakers have in producing L2 English liquids accurately.