Abstract Traditionally, age of acquisition (AoA) has been considered the single most important factor in second language (L2) acquisition and processing, particularly in the area of syntax processing. However, there is now growing evidence of the importance of other factors, such as the level of proficiency attained and the degree of overlap or similarity between the first language (L1) and L2 structures and possibility of transfer of features and/or processing routines. However, the relative importance of these factors and the nature of L1–L2 transfer are still unclear. To shed light on these issues, we recorded the electrical brain activity of a group of Chinese proficient late learners of Spanish, using the Event Related Potentials technique, while they read Spanish sentences containing violations of number and grammatical gender agreement (adjective–noun agreement and article–noun agreement). Unlike Spanish, Mandarin Chinese is an isolating language in which morphosyntactic features such as gender and number are not computed and so the ERP results from this group can help to clarify the role of L1–L2 transfer in morpho-syntax processing routines. The results included P600 effects for both gender and number agreement violations, with no differences between these disagreement conditions. These results are taken to support second language acquisition models which stress the roles of proficiency and L1–L2 transfer in L2 syntax processing.