This study investigates the challenges faced by Arab adult learners of English as a foreign language (EFL) in acquiring English restrictive relative clauses (RRCs), as well as the factors that affect the process of acquisition. This issue has received considerable attention in second language (L2) research. The present study discusses the syntactic structure of RRCs in English and Arabic with regard to the use of resumptive pronouns and the use of overt versus covert relative markers as related to the definiteness of the head noun. This study is carried out using an acceptability judgment test as the tool for data collection. A sample of 100 male and female adult Arabic speakers is used with the aim of identifying potential acquisition problems. The results obtained provide further support for first language (L1) transfer. This study concludes that participants accepted the use of resumptive pronouns and preferred the overt relative markers determined by definiteness. It is further discovered that certain factors influenced the acquisition process such as participants' age, age of learning, and age of immersion. The current study has some pedagogical implications for the teaching of relative clauses in the EFL context.