Using event-related potentials (ERPs), we investigated the impact of a range of individual difference measures related to L2 learning on proficient L1 Spanish – L2 English bilinguals’ brain responses during L2 morphosyntactic processing. Although grand mean ERP analyses revealed a biphasic N400–P600 response to English subject–verb agreement violations, subsequent analyses showed that participants’ brain responses varied along a continuum between N400- and P600-dominance. To investigate this pattern, we introduce two novel ERP measures that independently quantify relative brain response type and overall magnitude. Multivariate analyses revealed that larger overall brain responses were associated with higher L2 proficiency, while relative brain response type (N400 or P600) was predicted by a coalition of variables, most notably learners’ motivation and age of arrival in an L2 environment. Our findings show that aspects of a learner's background can differentially impact a learner's overall sensitivity to L2 morphosyntax and qualitative use of linguistic cues during processing.