This paper attempts to provide a reliable description of the characteristics of intrasentential language mixing produced by a group a Korean–English bilingual children, with a special focus on the distinction between code switching and borrowing. Making use of the inherent variability in case marking in Korean, this study employs a quantitative variationist method to determine the status of single nouns of English origin in an otherwise Korean discourse, which constitute the largest portion of the bilingual data. Analysis of the overall bilingual data suggests that intrasentential language mixing is determined by the bilingual abilities and preferences of the speaker as well as those of the addressee. The results of the variable analysis of case marking show that most of the English-origin objects and subjects of Korean verbs are best treated as borrowings and not as code switches.