While normal child language acquisition results in complete productive and comprehension abilities at a relatively young age, adult language acquisition is more belabored and often results in linguistic abilities that differ from those of native speakers in terms of both productive and comprehension abilities. A major line of research in language acquisition studies seeks to determine and explain potential causes of these differences. One current and influential hypothesis, the Interface Hypothesis (Sorace 2011), claims that dissimilarities are a product of the increased cognitive load experienced by bilinguals. That is, since bilinguals possess two grammars, they must simultaneously inhibit one language while activating the other. As a consequence of the increased cognitive load, the Interface Hypothesis predicts that bilinguals will show differences on properties requiring integration of purely linguistic information (e.g. syntax) with extralinguistic information (e.g. discourse or pragmatics). I test this prediction in near-native adult second language acquirers of Spanish who are native speakers of Farsi using offline tasks measuring grammatical accuracy and interpretation of referential and expletive subject pronouns as well as online tasks measuring language processing. The particular participant group, linguistic properties and combination of offline and online experimental methodology are especially relevant to the debate surrounding near-native speaker ultimate attainment potential and bilingual processing. That is, the experimental group participants meet the requirements of the Interface Hypothesis in that they are highly fluent second language speakers that have lived in a Spanish-speaking environment for over 30 years. While the language pairing is quite novel, the linguistic property examined is interesting in that Spanish and Farsi are both null subject languages with analogous discourse distributions of subject pronouns. However, the underlying syntax of each language differs therefore necessitating restructuring in order for the speakers to converge on the syntax of subjects in Spanish. Finally, employing both offline and online measures makes it possible to test the Interface Hypothesis's claim that bilingual processing is different from monolingual processing. The results, which provide mixed evidence regarding the claims of the Interface Hypothesis in that differences were found, but they were not isolated to overt subject pronouns, inform the field of linguistics regarding the human capacity for language acquisition, but also have significant implications for psycholinguistics, cognitive science and language teaching. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]