Problems in appropriately using the article system in English are globally known simply because they are observed in learners with different L1 backgrounds. The present study is dedicated to an investigation of the nature and degree of such problems when it comes to using articles for mass nouns by Persian-speaking learners. Data is elicited and analyzed from three proficiency groups. It is shown that even highly advanced learners do not have native-like mastery over the issue. It is also shown that L1 transfer is present in many cases of error. Explicit contrastive instruction based on the two languages is suggested as a more efficient way out. The results are also discussed within the framework of the Interpretability Hypothesis as they do not seem to support it.
