Interleaving, a surface configuration in some languages in which the parts of two adjacent words are interspersed with each other, has been argued to be a phonological phenomenon. In this paper, I investigate interleaving in Vietnamese and a related language, Pacoh (Katuic, Mon-Khmer), and argue that it is the result of morphosyntactic operations and structures and not a phonological operation. I present three pieces of evidence that interleaving is morphosyntactic in nature: (i) interleaving cannot apply to all syllables, only those in certain morphosyntactic environments; (ii) interleaving manipulates polysyllabic units and can apply to 3-part compounds, showing that it is manipulating morphosyntactic structure and not phonological structure; and (iii) interleaving creates extra syntactic-semantic force, suggesting a change in the syntax. I propose an analysis in which interleaving is the result of the structure of coordinate compounds, whose members have no precedence relation with each other, in combination with an alternate traversal of the syntactic tree during linearization.