In this article we investigate the effects of first language (L1) on second language (L2) neural processing for two grammatical constructions (verbal domain dependency and grammatical gender), focusing on the event-related potential P600 effect, which has been found in both L1 and L2 processing. Native Dutch speakers showed a P600 effect for both constructions tested. However, in L2 Dutch (with German or a Romance language as L1) a P600 effect only occurred if L1 and L2 were similar. German speakers show a P600 effect to both constructions. Romance speakers only show a P600 effect within the verbal domain. We interpret these findings as showing that with similar rule-governed processing routines in L1 and L2 (verbal domain processing for both German and Romance speakers), similar neural processing is possible in L1 and L2. However, lexically-driven constructions that are not the same in L1 and L2 (grammatical gender for Romance speakers) do not result in similar neural processing in L1 and L2 as measured by the P600 effect. (Contains 7 figures and 1 table.)