While the properties of bilingual codeswitching are well-documented, and switching morphology within words is often addressed, the properties of intraword phonology are less-understood. Morphemes from more than one language occur within a single word (e.g., a root from one language and affixes from the other). Furthermore, the affixes come only from the language which generates the syntactic tree while the root can be taken from either language. The data also strongly suggest that within such a morphologically mixed word, the phonology does not switch, a property I refer to as phonological uniformity. The key question explored in this paper is: why is phonological mixing in a morphologically mixed word not allowed? I first present evidence that there is phonological activation of both languages even in a monolingual task such as a Lexical Decision Task or silent reading which is consistent with an integrated I-phonological grammar. I provide a reanalysis of some fascinating data from Delgado et al. (2022) in arguing against a phase-based account of phonological uniformity. The mechanism which ensures that the phonology of the X0 matches the language of the affixes is Match Theory (Selkirk, 2011). The preferential mapping is between (a) syntactic phrases (XPs) and phonological phrases (f), and (b) syntactic heads (X0s) and prosodic words (w). Match Theory’s (monolingual) assumption that syntactic and phonological structure are isomorphic can easily be extended to bilinguals through language tags (Green & Abutalebi, 2013). I recast the Match Theory machinery within the framework of Co-phonologies (Sande, Jenks & Inkelas, 2020). In order to account for the differential behaviour of determiners in codeswitched simple DPs (e.g. the mesa) versus codeswitched complex DPs (e.g. the brown mesa) I show how a combination of the notion of the head of the phonological phrase, the free (as opposed to affixal) clitic status of the English determiner, and the parsing of an English vocabulary item via the Spanish contrastive hierarchy explains the phonological properties observed. A Null Theory phonological account of phonological uniformity is argued to be preferred over a phase-based account.