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.