This study explores how Arab L2 learners of English acquire mass nouns. The mass/count distinction is a morphosyntactically encoded grammatical distinction. Arabic and English have different morphosyntactic realisations of mass nouns. English mass nouns take the form of bare singular whereas Arabic mass nouns can take the definite singular form or the indefinite singular, but never the bare singular form. Therefore, the study explores how Arab learners interpret English mass nouns in light of the morphosyntactic differences between the two languages. 45 upper- and lower-intermediate Arab English learners were given a context-based acceptability judgment task on English mass nouns. It was hypothesised that Arabic learners would be influenced by their first language (L1), causing them to over accept definite singulars and under accept bare singulars as grammatical in mass noun contexts. The findings are consistent with what was hypothesised, except that Arab learners were found to interpret bare singulars accurately. It is argued that learners' performance is affected by not only L1 transfer but also UG accessibility where learners can structure away from L1 and more towards L2. Consequently, the findings implicate that L2 teachers should not teach grammatical structures that come for free and instead they should focus on grammatical structures that cause L2 acquisition difficulty.