This paper investigates the acquisition of prepositional relative clauses in L2 Spanish by English and Arabic speakers to understand the role of previous linguistic knowledge and Universal Grammar on the one hand, and the relationship between grammatical knowledge and its use in real-time, on the other. An oral production task and an on-line self-paced grammaticality judgment task were analyzed. Results indicated that the acquisition of oblique relative clauses is a problematic area for L2 learners. Divergent results compared to native speakers in production and grammatical intuitions were found; however, L2 reading time data showed the same real-time effects that native speakers had, suggesting that the problems with this construction are not necessarily linked to processing deficits. These results are interpreted as evidence for the ability to apply universal processing principles in a second language, and the relative independence of the processing domain and the production system.