The DEFINITENESS feature in English is both LF and PF interpretable while Persian is a language in which this feature is LF-interpretable but PF-uninterpretable. Hence, there is no overt article or morphological inflection in Persian denoting a definite context. Furthermore, Persian partially encodes specificity not DEFINITENESS. In DEFINITENESS both the speaker and hearer are involved while in specificity just the speaker may be taken into consideration. The associated indefinite article identifies an individual from a set but lacks the uniqueness feature. Specificity, on the other hand, may be defined in relation to the speaker. It signals the speaker’s intention to refer to some individual with a noteworthy property. Based on the predictions made by the interpretability hypothesis, it is predicted that Persian learners of English should be able to acquire the English DEFINITENESS feature lacking in their L1. To test the hypothesis, fifty L2 learners at intermediate and advanced levels were selected. To test their comprehension as well as production, they were given forced-choice elicitation and translation tasks.The results of the study showed that the L2 learners acquired the DEFINITENESS feature giving credence to the interpretability hypothesis in which the acquisition of LF-interpretable but PF-uninterpretable features does not pose a persistent problem. In the oral production task, the results show that the L2 learners use some compensating mechanisms such as demonstrative and possessive adjectives to encode DEFINITENESS. The results further indicate that the Persian learners of English experience some fluctuations in teasing apart the DEFINITENESS and specificity in indefinite referential singular contexts.