The aim of this study was an attempt to investigate the use of conjunctions in argumentative essays written by English as a Foreign Language fourth-year undergraduate Libyan students majoring in English at Omar Al-Mukhtar University in Libya. A corpus of 32 argumentative essays was collected from a sample of 16 students in order to be investigated in terms of Halliday and Hassan's (1976) taxonomy of conjunction. Findings showed that the Libyan EFL students used the conjunctions inappropriately, and that the adversative conjunctions posed the most difficulty for the learners, followed by additives and causals. Of the adversatives, "on the other hand" was the most difficult conjunction for the participants, followed by but and in fact. With the use of additive conjunctions, "moreover" was the most problematic, followed by "and" and "furthermore". Among the causals, the conjunction "so" was the most challenging, followed by "because". The findings of this study confirm previous studies that learners of English as a foreign language have difficulty in using conjunctions in their writing. The difficulties encountered by participants in employing the conjunctions can be attributed to three reasons: 1) first language (Arabic) negative transfer; 2) overgeneralisation in the second language (English) and 3) the presentation of conjunctions in lists in ESL/EFL textbooks without showing the subtle difference between them in terms of semantic function. These findings are discussed in this paper with implications for teaching the use of conjunctions in the Libyan context.