| Interlanguage, a linguistic system that combines features of the first language (L1) and second language (L2), is commonly observed among L2 learners and bilingual speakers. In phonology, interlanguage refers to an intermediate sound system that emerges as learners attempt to produce L2 sounds while still being influenced by their L1. This study explores the potential presence of interlanguage by investigating voice onset time (VOT) produced by L1 English–L2 Indonesian speakers. Through VOT measurements of the Indonesian voiceless stop consonant /k/ produced in monolingual and code-switching modes during spontaneous speech, this study aims to provide insights into how language learners and bilingual speakers use their L1 and L2 in naturalistic settings. Five adult L1 English speakers participated in the study, and their VOT values were measured using the Praat computer program. The findings indicate that speakers who acquired L2 (Indonesian) after the proposed critical period (i.e., a sensitive period for language acquisition) produced an interlanguage pattern in monolingual speech, with VOT values falling between their L1 and L2 norms. During code-switched speech, an interlanguage pattern was also observed, with VOT values falling between their L1 and their L2 production. Meanwhile, one speaker who learned the L2 before the end of the critical period produced native-like VOT values in both monolingual and code-switching modes. These findings contribute to research on L2 speech produced by language learners and bilingual speakers. The study also offers pedagogical implications for Indonesian phonological acquisition, particularly for L1 speakers, by highlighting the role of age of acquisition and L2 experience in VOT production in bilingual speech. |