This paper claims that lexical categories (V, N) and functional categories (D, I, C) behave differently in bilingual code-switching: whereas functional heads always determine the order of their code-switched complements, lexical heads may not do so. This proposal thus deviates from many recent studies which suggest that all heads determine the order of their complements (e.g. Mahootian, 1993; MacSwan, 1999; Nishimura, 1997; Nishimura and Yoon, 1998). Assuming a "Null Theory" perspective (Mahootian, 1993; MacSwan, 1999), code-switching data are explained here in terms of existing syntactic apparatus which also governs monolingual syntax. It is proposed that word order between lexical categories and their complements is determined by head parameter instead of feature strength as an intrinsic property of the lexical heads. Nonetheless, head-complement order is inherently specified in functional categories. On this account, prepositions are functional heads instead of lexical heads. © 2007 Elsevier B.V. All rights reserved.