This study tests the interface hypothesis by investigating how advanced Arab learners of English develop the grammatical knowledge of English indirect questions. Ten advanced Arab learners of English and four native speakers of English are tested on a written grammaticality judgment task and an oral production task. Three hypotheses are tested: (1) the grammatical knowledge of advanced Arab learners of English indirect questions is different from that of native speakers of English, (2) advanced Arab learners of English do not have problems acquiring indirect questions, where the matrix clause is a non-interrogative clause and the embedded clause is an interrogative clause, (3) advanced Arab learners of English have problems acquiring indirect questions, where both the matrix clause and the embedded clause are interrogative. The results of the study confirm the first and third hypotheses but disconfirm the second hypothesis. The results of the study provide counter evidence to the interface hypothesis, as even linguistic properties that do not involve interface between syntax and discourse/pragmatics seem to be persistently problematic at the advanced level of L2 proficiency.