This study examines the effect of proficiency and similarity between the first and the second language on grammatical gender processing in L2. In three experiments, we manipulated gender agreement violations within the determiner phrase (DP), between the determiner and the noun (Experiment 1), the postposed adjective and the noun (Experiment 2) and the preposed adjective and the noun (Experiment 3). We compared the performance of German advanced learners of French to that of French native controls. The results showed a similar P600 effect for native and non-native speakers for agreement violations when agreement rules where similar in L1 and L2 (Experiment 1, depending on proficiency), whereas no effect was found for L2 learners when agreement rules varied across languages. These results suggest that syntactic processing in L2 is affected by the similarity of syntactic rules in L1 and L2.