Each human language possesses a distinctive set of syntactic rules, and early, balanced bilinguals must learn two syntactic systems. The organisation of these systems in the bilingual brain is not yet clear; do they remain autonomous, or do they interact? This thesis examines the extent to which bilinguals’ knowledge of syntactic rules are co-active during monolingual sentence processing. Thus, the primary objective is to assess (a) whether bilinguals co-activate idiosyncratic syntactic rules, (b) how syntactic co-activation occurs, and (c) when syntactic co-activation occurs, focusing on contextual constraints. To this end, I manipulated English sentences according to the Welsh rules of soft mutation (a morphosyntactic process that alters the initial consonant of words), such that English sentences included ‘mutated’ (e.g. prince brince) or ‘aberrant’ (e.g. prince grince) nonwords, presented either explicitly or implicitly. In Chapters 3 and 4, syntactic co-activation led to the modulation of the phonological mismatch negativity (PMN), but only in sentences that would elicit a mutation in Welsh. Crucially, processing of explicitly processed nonwords was not influenced by lexical overlap between languages, indicating that bilinguals co-activate abstract syntactic rules during sentence processing. In Chapter 5, eye-movements were measured to determine the extent to which syntactic co-activation occurs in natural sentence reading (in which manipulated target words were implicitly processed). Syntactic co-activation manifested on later processing measures, reflected in longer reading times. Interestingly, this effect was restricted to trials in which there was lexical overlap between languages, suggesting that co-activation is sensitive to a lexical boost effect. Based on these findings, I propose a model of syntactic co-activation that is constrained by contextual demands: syntactic co-activation can occur via abstraction of syntactic rules, but may also be reliant on cross language lexico-syntactic associations during certain contexts.